North American Freight Train Brakes

By Al Krug

This page was written by Al Krug some years ago and was slightly edited by me and Peter Hinckley.  It is an excellent introduction to the current state of pure air brake design as used in North America and on heavy freight railroads.  If you would like information about other train braking systems, try our Brake Glossary, Air brakes, E-P Brakes, Vacuum Brakes or ECP Brakes.  


Contents

The Basic Equipment - Brake Application - Brake Release - Successive Applications - Emergency Braking - Emergency Reservoir - Locomotive Brakes - Quick Service Reservoir (Inshot) - Speeding Up Release - Retainers - Empty/Load Sensors - Equalising Reservoir - Self Lapping Brake Valve - Air Pressure Variations - Dynamic Brakes

If you would like more information about other aspects of train braking try our Brakes Page.

The Basic Equipment

Each freight car has an air tank (reservoir) on it. This reservoir (often called the auxiliary reservoir or brake reservoir) is charged with compressed air from the locomotive's air compressor thru the train line brake pipe. You can see the air hoses between the cars and they go Kapowssssh when you uncouple them. After the initial charging of the reservoirs, the brakes can be set (applied) by REDUCING the pressure in the brake pipe. Compressed air from each car's reservoir pushes on the brake cylinder piston to apply the brakes on the car. In the rare event that you have no air in the reservoir then you've got no brakes and you've got a run away.

Now if you are not technically oriented or don't understand the meaning of psi (pounds per square inch air pressure) you may as well give up right here and be satisfied with the above description. If you understand compressed air and have a high boredom threshold then read on.

OK. How are the brakes controlled? That is the job of the so-called "triple valve" on each car. Basically this valve compares brake pipe air pressure and car auxiliary reservoir air pressure. If the brake pipe pressure is HIGHER than the reservoir pressure, the triple valve moves to the RELEASE position. In this position it vents any brake CYLINDER air to atmosphere thus releasing the brakes. It also connects the BRAKE PIPE to the RESERVOIR so brake pipe air pressure can begin recharging the reservoir. This is the situation you are in when you are CHARGING the brake system sitting in the yard waiting for a brake test ("pumping up the air").

So now the locomotive's diesel engine is turning an air compressor that is pumping air thru my brake valve into the brake pipe of the train, back along the train and thru the triple valves of each car into the reservoirs. This takes a lot of air. It takes anywhere from 15 minutes to an hour to charge a train depending on its length and how leaky the air hose couplings are. On the railroad I work for the standard brake pipe pressure is 90 psi.

Brake Application

Once the cars' reservoirs are charged to the same pressure as the brake pipe (90 psi), the brakes are ready to be used, either on the road or for an air brake test.  Now suppose I want to set the air brakes. What I do is move the brake valve handle from the RELEASE & CHARGE position to the APPLICATION position. This disconnects the loco's air compressor from the brake pipe and opens a small hole to vent brake pipe air pressure to atmosphere. This venting of brake pipe air causes the brake pipe pressure to drop slowly.

On each car the triple valve is "watching" the brake pipe pressure and the reservoir pressure. Remember when the system was charged, both reservoir and brake pipe were at 90 psi. Now with the brake pipe pressure dropping, the triple valve senses that the brake pipe is LOWER than the reservoir. This is a signal to the triple valve on that car to move to the APPLY position. In the apply position it connects the RESERVOIR air pressure to the BRAKE CYLINDER. This air pressure pushes the brake cylinder piston out and applies the brakes.

Meanwhile, up in the cab, I watch my gauges and when I get the brake pipe pressure lowered to where I want it, I put my brake valve in neutral or LAP.  Lap simply seals the brake pipe, letting no more air out and not letting any air compressor air into it.

Let's say I "made a 10 pound set".  This means I reduced the brake pipe air pressure from 90 psi to 80 psi then "lapped" the brake valve.  Remember the triple valve on the car was watching the brake pipe air pressure and, as soon as it dropped below reservoir pressure, it moved to the APPLY position and allowed reservoir air to flow into the brake cylinder.  This flow of air from the car reservoir to the cylinder will of course lower the pressure in the car reservoir. Remember that the triple valve always watches the pressure in the brake pipe and the reservoir.  It allows air to flow from the reservoir into the brake cylinder UNTIL THE RESERVOIR PRESSURE LOWERS TO THAT OF THE BRAKE PIPE PRESSURE.  Now once again the reservoir and brake pipe are equal pressure so the triple valve returns to its own LAP position. Both now have 80 psi.

But now, all that air that flowed from the reservoir to the cylinder has applied the brakes on that car.  The volume of the reservoir is about 2.5 times the volume of the brake cylinder.  So to lower the reservoir 10 psi, from 90 to 80, enough air flowed from the reservoir into the cylinder that it put 25 psi (2.5 times 10 psi reduction = 25 psi) in the brake cylinder.  Simple isn't it?

I now have the choice of leaving the brakes applied to control speed or stop, or I can make another reduction to get heavier braking, or I can release them. Let's say I want to slow down faster than I am so I want more braking. I move my brake valve to the application position and lower the brake pipe another 5 psi from 80 to 75 psi. The triple valves on the cars sense once again that the brake pipe (75psi) is LOWER than the reservoir (80psi). So they move to the APPLY position and allow more reservoir air to flow into the cylinder until the reservoir is the same pressure as the brake pipe (75psi). The brake cylinder pressure goes up and so the braking effort goes up.

Because of the 2.5 ratio of reservoir volume to cylinder volume this 5 psi reduction results in (2.5 times 5psi =) 12.5 psi more braking pressure. This is on top of the 25 psi already there for a total of 37.5 psi brake cylinder pressure. Notice that if this air brake system has been fully charged, it is "fail safe". That is, anytime the brake pipe air reduces, the brakes apply. Thus if a train comes uncoupled or an air hose bursts, the brakes apply fully, automatically. The amount of braking relies on the amount the system is charged however.

Brake Release

When I no longer need the brakes I can release them. This is done by moving my brake valve to the RELEASE & CHARGE position. As before, this simply connects the locomotive air compressors to the brake pipe and pumps air back thru the brake pipe raising its pressure back to 90 psi. The cars' triple valves sense that the brake pipe (90psi) is HIGHER than the reservoir (75psi) and move to RELEASE position. This connects the brake cylinder to the atmosphere, releasing the air pressure in the cylinder and thus releasing the brakes. It also connects the brake pipe to the reservoir to begin recharging the reservoir from the brake pipe. Notice there is NO GRADUAL release; the release is a complete release.

Successive Applications

You now know the basics of air brakes. But as always in life there are complications. First of all, note that when the brakes released on the train cars, the brake pipe was at 90 psi but the reservoirs were at 75 psi. Upon releasing, the reservoirs BEGIN to recharge but that takes time. So for several minutes after releasing the brakes, the reservoirs are not fully charged and thus I do not have full braking available.

Suppose I had made a total reduction of 15 psi as above, this reduced the brake pipe and reservoirs from 90 to 75 psi. I then release the brakes by raising the brake pipe back to 90 psi. Suppose 1 minute later I want to set the brakes again. The brake pipe is at 90 psi but the reservoirs may have only re-charged from 75 psi to 79 psi! Now if I make a 10 psi reduction of the brake pipe from 90 to 80 what does the car triple valve see? It sees 80 psi in the brake pipe and 79 psi in the reservoir. The brake pipe is HIGHER than the reservoir so it stays in the release position! I get NO brakes!

IF I reduce a further 5 psi to bring the brake pipe down to 75psi, the triple valve sees the brake pipe LOWER than the reservoir (79) so it goes to APPLY position. This once again allows reservoir air to flow into the brake cylinder until the reservoir pressure lowers to the brake pipe. The brake pipe is at 75 psi and the reservoir was at 79 psi so the reservoir lowers 4 psi. The 2.5 volume ratio between the reservoir and brake cylinder means I got (2.5 times 4 psi =) 10 psi in the brake cylinder. Very little brakes where before with the same 15 psi reduction I got 37.5 psi in the cylinders! This is how most classic runaways occur. Imagine going down a long mountain grade, a dumb engineer makes several heavy sets and releases of the air brakes in a short period of time. He soon finds he has no brakes because there is very little air left in the reservoirs. This is known as "pissing away your air". Now before you go tell the press at the scene of a run away train wreck that it had a dumb engineer allow me to state that there are other ways runaways can occur that are no fault of the engineer.

Emergency Braking

Another complication of this simple brake system is that a long train has a long brake pipe and all that pipe contains a lot of air. When I want to make a brake application by reducing the brake pipe pressure, it takes TIME to vent enough air to lower the pressure to where I want it. This can be managed under normal braking conditions but what about emergencies? They solved that by adding an emergency vent valve to each car. This valve watches the brake pipe air pressure. If the brake pipe pressure goes down SLOWLY the emergency valve does nothing, no matter how low the pressure goes. But if the pressure drops QUICKLY the emergency valve senses this and opens the car's brake pipe to the atmosphere. This quickly dumps the brake pipe air to the atmosphere at the car.

In other words all the air does not have to go thru the entire brake pipe up to my valve in the loco for emergency. All I have to do is START the emergency application by venting brake pipe air at the head end QUICKLY. This causes the first car emergency valve to sense the fast drop and move into emergency. This vents all brake pipe air at that car quickly, the next car senses a fast drop and also goes to emergency, then the next and next and so on. Within seconds the entire train is in emergency, dumping all the brake pipe air at each car. You get a fast and full application of the brakes through out the entire train.

If you are standing near a train when the loco uncouples you can hear these emergency valves vent the brake pipe pressure locally on the car you are next to. That car will go "Psssssht". If you are standing some distance off to the side of the train you can hear each car trigger in succession as the "psssht, psssht, pssht, pssht" goes rapidly back the train. These emergency vent valves stay open for about 2 minutes after triggering. This ensures the train is stopped before you can release the brakes. Anything that causes a QUICK drop in brake pipe pressure at any car, will trigger that car which in turn triggers adjoining cars and thus puts the whole train in emergency. This initial trigger could be the engineer, the conductor pulling his emergency valve in the caboose, the brakeman pulling his valve in the cab, the train or air hoses coming uncoupled, or an air hose bursting.

Emergency Reservoir

This is all well and good in theory but what about that doofus engineer described above that "pisses" away his air so there is little air in the reservoirs? He still gets very little braking in emergency, he just gets it quicker. To ensure that there is always air pressure on each car for an emergency application, they modified the basic system. They added a second reservoir to each car! The original reservoir we've discussed up to this point is called the SERVICE RESERVOIR because it is the one used in normal service braking.

The new reservoir is called the EMERGENCY RESERVOIR because it is only used in emergencies.  It is installed in the "AB" type brake equipments.  "AB" stands for "Automatic Balancing"; and "automatic" service portion and a "balanced" emergency portion.  This emergency reservoir is charged with compressed air from the brake pipe just like the service reservoir. After the initial charging time in the yard, it has 90 psi in it.  

Anyway, if I initiate an emergency application by making a QUICK reduction at my brake valve, each car's emergency valve triggers just like before, but now it also connects the emergency reservoir air to the brake cylinder in combination with the service reservoir air.  This ensures that there will be air pressure for an emergency stop.

By the way, when I make a service application of the brakes, I vent the brake pipe air thru a SMALL HOLE in my brake valve. This lowers the brake pipe air pressure slowly. When I want an emergency application of the train brakes I move my brake valve over to the emergency position. This is just a BIG HOLE that allows air to escape QUICKLY and that triggers the emergency valves on the train cars. That is where the terms "big hole 'em" and "He went to the big hole" come from, meaning an emergency application. Also the term "Dump the Air" means go into emergency. It comes from the fact that you initiate an emergency application and cause each cars' emergency valve to "dump the air" locally. Regular service applications of the brakes are referred to as "set em up", "set the brakes", "set the air", "squeeze the breeze".

Locomotive Brakes

The locomotives have air brakes just like the cars and they will apply when the brake pipe air pressure is reduced just like the car brakes. This is not always desired, especially when stretch braking with the throttle open and car brakes set to control the slack action. The engineer can prevent the loco brakes from applying at these times by depressing the independent brake handle and holding it down. This is called "bailing off the air", or more correctly, "actuating off the air".

Locos also have "independent" brakes that you can apply on the locos only. This is straight air where the air pressure comes straight from the main air compressor reservoirs on the locos to the locos' brake cylinders. It is controlled by the position of the independent brake handle. It is used to apply the loco brakes only for switching or for holding a stopped train on level or low grade track. Of course each loco also has an air compressor & main air reservoirs on it. They are all connected by hoses to the lead loco so all units can help supply air. These main reservoir hoses and independent brake control hoses are the hoses you see between loco units. The big electrical jumper between units is a 27 wire cable that has control wires for trailing units' throttle, headlights, reverser, compressor control, generator field control, dynamic brake set up and control, engine alarm bell, sanders, etc.

You are now an expert on train brakes. There will be a quiz on Wednesday.

Quiz.

(Is it Wednesday already?) You have made a 10 psi brake pipe reduction on a fully charged train. The brakes have applied. One car has a leak in its service reservoir. What happens to the brakes on that car? What happens to the brakes on the entire train?

Answer: (no peeking) The key is to remember how the triple valve works. It senses the DIFFERENCE between the brake pipe pressure and the service reservoir pressure. If the brake pipe is higher than the reservoir the valve moves into release position. If the brake pipe is lower than the reservoir it moves into the apply position. If you have made a 10 psi reduction from 90 to 80 psi and the brakes have set, the reservoir and brake pipe are both now at 80 psi. As the reservoir slowly leaks the pressure drops, from 80 to 79 to 78 to 77 etc. As soon as the reservoir pressure leaks from 80 to 79 psi the triple valve "sees" that the brake pipe (80) is HIGHER than the reservoir (79). IT WILL MOVE TO THE RELEASE POSITION and release the brakes on that car!!! Whoa! This is not good. But you still have brakes on the other 109 cars of the train, hopefully.

Quick Service Reservoir (Inshot)

Because, during a normal service application, all the brake pipe air has to vent thru the loco's control valve, it takes a long time to get the brakes set through out the train.  The pressure first drops near the front of the train and then drops further and further towards the rear.  This causes the brakes to set up on the front part of the train before they set on the rear portion. This causes the rear end to run into the front end (slack action).

So some smarty comes up with the idea of modifying the triple valve on the cars so that when a car first senses a drop of pressure, it opens a passage from the brake pipe to a small reservoir (a third reservoir called a "quick service reservoir").  This reservoir is sized such that it has the proper volume in relation to the car's brake pipe volume that filling it with air from the brake pipe will reduce the brake pipe 6 psi.  That means when I make anything up to a 6 psi reduction, 6 psi worth of it is done at each car.  This results in a faster (but not fast enough to trigger emergency) more even application of the brakes thru the train.  It only works the first time, however, since after that the reservoir is filled and remains so until the brakes are released.   See also Footnote 1 below.

Speeding Up Release

Because of the long brake pipe of a train and all those cut off valves at the ends of each car and other restrictions, it takes time to pump air back thru the train to release the brakes. As a matter of fact, as each car goes into release it begins recharging its reservoir from the brake pipe, consuming air from the brake pipe further slowing down the build up of pressure towards the rear. This results in the head end releasing first and causes problems with slack action as the front portion running free runs away from the rear portion still braked.

In the early days they solved this problem by putting chokes in the pipes that carried air from the brake pipe to the reservoir on each car. This allowed a more rapid build up of air pressure in the brake pipe all the way to the rear of the train since each car reservoir was consuming brake pipe air at a slower rate due to the choke restriction as it recharged following a release. But as trains got longer and heavier even this was not enough and the chokes slowed down the initial charging of the trains in the yard and the recharge on the road. So along comes Mr. Smart again. Like a Congressman and the social security fund, he can't stand to see a surplus go unused.

Remember that emergency reservoir they put on each car? It was initially charged to 90 psi and never used if the engineer did not need an emergency application. All that air there. They modified the triple valve again so that when a car goes into release it: 1) vents the cylinder air to atmosphere releasing the brake as before, 2) connects the brake pipe to the reservoir to begin recharging as before, and now 3) connects the 90 psi emergency reservoir to the BRAKE PIPE to boost the brake pipe up quickly at each car. This results in fast releases thru out the train length, but it depletes part of the emergency air available if you need it right away before it can recharge. Basically that's how train brakes work today.

Quiz #2

With a whole train of this type of equipment, what are the answers to the two questions posed in quiz #1 with under the same conditions?

Answer:

Same as before. The brake will release on the leaky car. However, with this type of equipment that one leaky car will dump its emergency reservoir air into the brake pipe when it moves to the release position. This action will raise the brake pipe pressure on that car AND THE CARS NEXT TO IT! When the cars next to the leaky one see the brake pipe rise slightly above their service reservoir pressure, their valves interpret this as a release signal AND THEY ALSO MOVE TO RELEASE! Now they also dump their emergency reservoir air into their brake pipes and that triggers the cars next to them to release. Because of that one leaky service reservoir the entire train will release. It is for this reason that it is against the rules to "bottle the air", close the angle cock on the train, when uncoupling the engines.

Some Other Embellishments

Retainers

On long steep grades it may be necessary to set and release the brakes several times due to grade changes etc. But, if you release the brakes on a steep hill the train immediately accelerates. If you immediately reset the brakes you get less braking than before because the car reservoirs have not had time to recharge. Because of the long recharge time on long freights a way was needed to keep the brakes applied on the cars yet allow them to recharge. Enter the retainer valve.

When a triple valve moves to release, it connects the reservoir to the brake pipe to begin recharging the reservoir from the brake pipe. It also vents (exhausts) the brake cylinder air to atmosphere to release the brakes. The retainer valve is mounted on the exhaust pipe of the brake cylinder and can restrict or close off that exhaust. This restriction holds some of the air in the brake cylinder, thus keeping the brake applied even though the triple valve is in release where it allows recharging of the reservoirs.

Retainer valves are completely manually operated, i.e. the train must be stopped (usually at the top of a long grade), the brakes released, and a crewman must walk back along the train. He turns the retainer valve on each car to the restricting position. Usually only a percentage of the cars are "retainered", just enough cars to keep the train from running away down the hill when the brakes are released and recharging during the run down the mountain. When the crewman is back aboard the train may proceed down the mountain. The air brakes work normally until they are released. Then the cars with the retainers closed will hold their brakes applied, slowing train acceleration while the reservoirs recharge for the next brake application. The train must stop at the bottom of the grade and a crewman again walk back and return the retainers to their open (direct release) position.

The retainer valves have four positions; direct release, slow release, low pressure hold, and high pressure hold. There are very few places in the U.S. today where retainers are used on a regular basis as dynamic brakes serve much the same purpose, controlling the train acceleration while the air brakes recharge. Since the dynamic brakes slow down the rate of acceleration the brakes have longer to recharge before they are needed again. Also the retarding effort of the dynamic brakes allow the engineer to use lighter air brake applications to control train speed, thus the car reservoirs are not depleted as much and require less time to recharge. However if a train should happen to go into emergency due to a burst air hose or such and stops on a long steep grade you would NOT want to release the train brakes after fixing the hose. The brakes would release completely and you will have almost no air remaining in the car reservoirs to reapply the brakes. If you don't have dynamic, or it is not sufficient to slow the train, then you'd have a run away. The solution is to walk back before releasing and turn on the retainers to hold the brakes on some cars. Then release the brakes and roll down the hill with these "retained" cars controlling the acceleration while the entire train recharges.

Empty/Load Sensors

Traction between the wheels and the rail is directly proportional to the weight on the wheels. The amount of traction determines the amount of braking that can be applied without sliding the wheels. Sliding wheels develop flat spots within a few feet. Train cars have a large weight difference between the loaded condition and the empty condition, especially modern coal hoppers and grain cars.

The maximum braking effort of a car must be designed so that when in emergency (when the highest brake cylinder pressure is obtained) the EMPTY car will not slide its wheels. Unfortunately this means a heavily loaded car is under braked even in emergency. A way was needed to allow higher brake cylinder pressures on loaded cars than on empty cars.

The first step was to put larger reservoirs on the cars so that the traditional 2.5 to1 ratio of reservoir volume to brake cylinder volume was greater. This will result in higher brake cylinder pressures for any given brake pipe reduction. The problem is that higher pressure will slide the wheels of an empty car. So a pressure limiting valve is attached to the brake cylinder which will vent any excessive pressure to the atmosphere thus limiting braking effort. The exhaust of this limiting valve is open to atmosphere on an empty car allowing it to vent excess pressure. On a loaded car it is closed off so it cannot vent any pressure from the brake cylinder thus taking advantage of the higher pressure which results in higher braking effort.

The closing or opening of the limiting valve exhaust is controlled by a load/empty sensing arm. The pressure limiting exhaust close off valve is mounted on the car frame just above the truck frame. One end of an arm is attached to the close off valve and the other end rests on the truck frame. If the car is EMPTY the car body rides high on the springs and the arm moves the close off valve to the open position allowing the limiting valve to vent excess pressure. A LOADED car rides low on the springs and the arm is pushed up, moving the close off valve to the closed position, thus preventing the limiting valve from venting the higher brake cylinder pressure.

Equalising Reservoir

I stated earlier that the engineer makes a service application of the brakes by moving his brake valve handle to the application position, opening a small hole, which reduces brake pipe pressure slowly. He watches the brake pipe pressure fall on the air brake gauge. When he gets the amount of reduction he desires, he moves the brake handle to the LAP (blocked off) position.

This method is true only for very early air brake systems. As trains got longer, thus more brake pipe volume, it began to take too long for the air to travel thru all the cars to vent at the engineer's brake valve. His attention was fixed on the air brake gauge far too long to be safe. So the locomotives had another small reservoir installed called an EQUALISING reservoir. This reservoir is very small compared to the brake pipe volume of a long train and thus its pressure can be reduced almost instantaneously. The engineer's brake valve now reduces the air in the equalising reservoir instead of the brake pipe. He can get the desired reduction (say 10 psi) very quickly and then can take his eyes off the equalising reservoir gauge to look out ahead. An equalising valve is connected between the equalising reservoir and the brake pipe and it is this valve that vents the brake pipe air to atmosphere until it reduces to be equal to the equalising reservoir pressure.  See also Footnote 2.

Self Lapping Brake Valve

Since the late 1950s or early 1960s the engineer's brake valve has been of the self lapping type. That is, he no longer has to move the brake valve back to the LAP position after making a reduction. The position of the brake valve handle determines the amount of reduction made.

Air Pressure Variations

The engineer can change the maximum pressure of the brake pipe by adjusting the FEED VALVE at his control stand. I have used 90 psi as the standard pressure to which the brake pipe is initially charged and subsequently recharged. On the railroad I work for, 90 psi is the standard. Some railroads use 80 psi as a standard. Some mountain grade railroads use 100 psi in mountain territory on loaded coal and grain trains.

What is the significance of these different pressures? During normal service braking operations there is none. A 10 psi REDUCTION from a 100 psi brake pipe, a 90 psi brake pipe, or an 80 psi brake pipe all result in 25 psi in the brake cylinder and thus equal braking effort. Remember that 2.5 to 1 ratio between service reservoir volume and brake cylinder volume.

But what happens if you make a 26 psi reduction from a 90 psi brake pipe? 90 minus 26=64 psi in the brake pipe. Remember the triple valve moves to the apply position and allows service reservoir air to flow into the brake cylinder until the service reservoir pressure lowers to equal the brake pipe pressure. As the service reservoir pressure flows into the brake cylinder the brake cylinder pressure rises. Because of the 2.5 to 1 ratio of volumes, when enough air has flowed into the brake cylinder to lower the service reservoir 26 psi the BRAKE CYLINDER PRESSURE IS 64 psi !!! (2.5 times 26 =64) This air came from the service reservoir which is NOW AT 64 psi ALSO. Since the reservoir pressure and the brake cylinder pressure ARE EQUAL no more air will flow into the brake cylinder.

This condition is called a FULL SERVICE brake application because even reducing the brake pipe further, below 64 psi, WILL NOT INCREASE the amount of brake cylinder pressure. Even if you reduce the brake pipe pressure to zero psi the reservoir and brake cylinder pressure will still be 64 psi., the same as it was with only a 26 psi reduction. This full service or "equalisation of pressures" occurs at 64 psi for a 90 psi charged system. It occurs at 71 psi for a 100 psi charged system resulting in higher full service brake effort. It occurs at 57 psi for an 80 psi charged system resulting in lower full service braking effort. The corresponding brake pipe REDUCTIONS are 26 psi for the 90 psi brake pipe, 29 psi for the 100 psi brake pipe, and 23 psi for the 80 psi brake pipe.

An engineer who makes a reduction greater than these values is just wasting time, no higher braking effort results. This is all academic however since normal train operations seldom require a brake application greater than a 15 psi reduction and any reduction greater than 12 psi is considered heavy braking. So why would mountain grade railroads use 100 psi in the brake pipe? Two reasons. First as we just saw, the full service braking effort IS higher if it is needed. Second, suppose a 10 psi reduction is made from a 100 psi charged system (100-10=90). This results in 25 psi in the brake cylinders (10 psi times 2.5).

Part way down the mountain the grade lessens and train speed drops. The engineer releases the brakes and the brake pipe returns to 100 psi. The train immediately begins to accelerate down the grade. He immediately resets the air brakes by making another reduction. But the car reservoirs have only just begun to recharge so they have only 90 psi in them. If he makes a 10 psi reduction of the brake pipe (100-10=90) he will get no brakes. This is because the brake pipe will be at 90 and the reservoirs are also 90. But if he makes an additional 10 psi reduction (a total of 20, 100-20=80psi) he will get the same braking effort as the original set, 25 psi in the cylinders. So you can see that the 100 psi charged system AFTER one 10 psi set & release is in the exact state a 90 psi charged system is in when fully charged. This means the 100 psi charged system gives him one additional 10 psi set and release before he begins to run out of air compared to the 90 psi charged system.

So why not use 100 psi? There are penalties that go along with that extra pressure. One is that any weak hoses or valve gaskets may fail at the higher pressures. Another is if the train should go into emergency for any reason the higher braking effort may be enough to lock up and slide car wheels, especially on empty or lightly loaded cars. This will cause wheel damage at the very least and possibly a derailment from failed wheels later. A third reason is it takes longer to charge a train initially to 100 psi instead of 80 or 90 and the higher pressures cause more leaks in the system.

So why the 80 psi system? Long ago, like in the 1920s, the brake pipe was 70 psi.  That was fine for the 40 ton cars of the day.  By the 1940s the coal cars had grown to 55 tons and the brake pipe pressure pushed to 80 psi. In the 1950s the cars were 70 tons and in the 1960s had grown to 100 tons.  Still 80 psi brake pipe pressure handled the braking chores OK.  By the 1970s coal & grain cars had climbed to 135 tons and the 80 psi brake pipe had little margin for error.  On unit coal trains of 15,000 gross tons, even on 1.25% grades.  The emergency stop distances for heavy trains was growing longer and longer.

During the 1970s our railroad rules dictated an 80 psi brake pipe for all trains EXCEPT loaded unit coal and grain trains which were to use 90 psi.  This shortened emergency stop distances for these heavy trains but created other problems.  For instance when the trains were unloaded the pressure had to be reduced.  If a coal train using a 90 psi pressure gave cars during switching operations to a freight using an 80 psi brake pipe, the "over charge" condition had to be reduced.  This didn't always get done properly resulting in stuck brakes on some cars and over heated wheels.  As the weight of lumber, tank, and other cars caught up to the coal and grain cars and load/empty sensors were applied the railroad simply mandated a 90 psi brake pipe for all trains.  However, railroads that don't operate unit coal trains or don't have steep grades still use 80 psi since it is adequate for their type of operations.  Some yard and transfer operations that operate at low speeds still use 70 psi, taking advantage of the shorter charging times.  See also Footnote 3.

Dynamic Brakes

Dynamic brakes are easy. Basically you just turn the traction motors into generators and turn the electric power they produce into heat and dissipate it. Contrary to popular belief the motors are NOT put into reverse!

Normally, while pulling (motoring) the traction motors are your standard DC motors. The output of the main generator is applied to the traction motor armatures and fields and they "motorvate". However when you change to dynamic braking, heavy duty contacts "re-wire" the motors. The ends of the field windings are connected across the main generator output so that the main generator is applying power only to the fields. The ends of the armature are connected across iron resistance grids. As the train moves down the track, wheels turn the traction motor armature. Since the armature is turning in a magnetic field, created by the field windings powered by the main generator, the armature generates electricity. This electricity flows thru cables up to the resistance grids, the grids get hot using up the electricity. Large blower fans cool the grids to keep them from melting. In principle, the whole thing works similarly for AC drives but the electrics are different.

It takes lots of power to turn those armatures to generate all that electricity being thrown away as heat. This power comes from the rolling train thus retarding it. Because the armatures must turn a minimum speed to generate power, you can not stop a train with dynamic brakes. You can only control its speed or slow it down. As you near 12 mph the armatures are turning slowly enough that they generate little power so braking effort drops off rapidly. At higher speeds the amount of braking is controlled by me, the hogger. I move the dynamic brake lever and that in turn controls the output of the main generator which is supplying the traction motor field current. The stronger the fields the more power generated by the rotating armature so the more braking effort you get. Simple ain't it?

Actually that is a pretty good description of how it works but in reality it is a little more complicated. Life always is isn't it. I don't really control the output of the main generator directly. My lever controls a rheostat that controls transistors that control the field of the exciter generator. The output of the exciter goes to the field of the main generator and that controls the output of the main generator, which goes into the traction motor fields so the rotating armatures can generate the electricity producing braking effort. Wheeze! Got it? Also various sections of the resistance grids are switched in and out of the circuit to provide different amounts of electrical load thus different braking forces.

About the only part of dynamic brakes you can see are the resistance grids and their cooling blowers. On EMD locos they are along the top of the roof of the long hood about in the centre of the loco. That is the "bulge" along the roof line with one or two 36" blower fans on top. Older GEs (U25, U30 era) have the dynamic brake grids mounted in the radiator cooling air intakes on the side of the hoods. You can see them if you look thru the screens. The grid cooling air is supplied by the single large radiator fan on these GEs. Newer GEs and EMDs have a boxy affair mounted high immediately behind the cab. These have their own grid cooling blower. You can tell when any loco is in dynamic braking going down hill because these blowers suck a lot of air and whine. Once in a great while a grid cooling fan will fail or a grid will short circuit.  This results in the iron grids actually melting, accompanied by molten slag blowing from the unit and all sorts of arcing and pretty sparks.  A great show at night.

Footnotes

1.  Quick Service

The Quick Service feature is an 1895 requirement of the Master Car Builders' Association (now called the AAR).  Early triple valves didn't have Quick Service.   The feature was first introduced with the K freight brake of 1900 (or 1901) and has been a requirement for all braking systems since.  Triple/control valves accomplish quick service in one of three ways, depending upon the model of the valve: put the air into the brake cylinder (K does this), throw the air away (AB/ABD does this), or save it in a reservoir for use if an emergency is called for later (a modification to ABD/ABDX for some designs of TOFC, auto rack, articulated container cars, and others).  This third method is the reservoir mentioned.

It takes only about 1 to 1½ psi pressure differential across the face of the triple/control valve operating piston to move it.  Once it moves, it goes to Quick Service, with no choice offered and reduces the brake pipe by 6 psi.  If the engineer makes an initial reduction that is greater than 6 psi, Quick Service action within the valves will be bypassed.  If he makes one that is less, the valves on the cars will make sure it becomes a 6 psi reduction.  The Inshot Piston is a device inside the Emergency Portion of an AB control valve and it is used to regulate the flow of air into the brake cylinders into two stages during an emergency application in order to get a modicum of control over slack action in the train during the emergency application.

2. Equalising Valve

There are a few automatic brake valves (mostly special passenger ones) that vent both the Equalizing Reservoir and the Brake Pipe to atmosphere at the same time.  This is done to eliminate the delay between the movement of the brake handle and the onset of reduction of brake pipe pressure, essentially ensuring "instant" automatic brake applications.

3. Air Pressure Variations

US automatic brake systems are still designed around the ratio of 2:5:7 and the 50 foot freight car.  When automatic brakes were first introduced, 70 psi was the operating pressure and this pressure was used by some railroads right up until almost 1960.  Taking 2/7 of this pressure out of the brake pipe results in a brake cylinder pressure of 5/7 of this value; a 20 psi reduction produces a 50 psi brake cylinder pressure.  This is the notorious "Point of Equalization" where further decrease of brake pipe pressure won't produce an increase of brake cylinder pressure. When the base operating pressure was raised to 90 psi (current US standard for freight), the numbers became 26, 64, and 90 ; for 110 psi (the current passenger standard) they become 32, 79, 110.  The ratio doesn't have units.  If you threw away the British based gauges used in the US and substituted metric ones (kgs per sq cm), used a self-lapping automatic brake valve to apply the brakes and measured the full service pressures, you will find that a full service reduction is still 2/7 of the base operating pressure.   The reason for the original 50 psi brake cylinder pressure standard revolves around the economics and practicalities of air consumption and replenishment and the geometry of the foundation brake rigging on the cars.

See also Brake Glossary, Air brakes, E-P Brakes, Vacuum Brakes or ECP Brakes.