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.
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.
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.
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.
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.
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.
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".
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.
(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.
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?
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.