Single Line Operation
Many railways around the world have more single track line than multiple track, so single line operation plays an important part in everyday train working and signalling practices. The normal practice is to provide a single line with passing loops or short double track sections to give trains running in opposite directions places where they can pass. This page gives an overview of the operation of trains over a single line and covers both US and UK practice.
More articles on this site about signalling are listed in the Signalling Index
One Engine in Steam - Single Line Operation in the US - The Timetable - Train Speed Rules (US) - Train Spacing - Train Orders - Track Warrants - Unsignalled Operations in the US - Centralised Traffic Control (CTC) - Single Line Operation (UK) - Staff and Ticket System - Electric Token Block - Radio Electronic Token Block (RETB)
One Engine In Steam
The simplest form of single line operation is where the line has only one train - so called "one engine in steam" operation. This is usually only practicable on a short branch line where the volume of traffic is low enough not to require more then one train. The entrance to the line is protected by a locked manual ground lever frame or remotely controlled track circuited block. In the UK, a "staff" or "token" may be issued to the driver as authority to proceed on to the single line.
Single Line Operation in the US
Fig.1: Diagram of typical single line with passing loops. In the US, a passing loop is often referred to as a siding. The length of the loop must allow for the accommodation of the longest train using the route.
Many railways around the world are operated where trains work in either direction over a single line. Trains running in opposing directions pass each other at places where loops or sidings are available (see diagram, left). Obviously, special, and somewhat elaborate precautions are used to prevent head-on collisions, or "cornfield meets" as they are called in the US.
The principal difference between UK and US operation of single lines is that the UK uses lineside signals whereas, over large sections of the country, the US does not. This is changing as more and more US lines come under centralised signalling control known as CTC (Centralised Traffic Control) but there are still huge areas of "dark territory", as it is called, which have no fixed signals. For these areas, special rules are required and they have developed, over the years, into a complex system of train dispatching called "timetable and train order" operation.
The basic authority for train movements over a single line in the US is the timetable. It is used as the first level of regulation for all trains. The timing and priority for trains is laid down in the timetable, i.e. the timetable dictates the departure times and then, if times cannot be adhered to, which train should move first and which should wait for others. There is normally a set of rules of priority, e.g. "superiority by direction" where trains going in one direction, say westbound, have precedence over trains going eastbound. In such a case, an eastbound train which arrives at a passing loop will always have to wait in the loop until a westbound train or trains has passed. Different types of trains are also given priority, passenger over freight, for example. Dispatchers, who are responsible for the movement of trains, always try to conform to the timetable first before issuing a train movement order.
Train Speed Rules (US)
In the US, train speeds are regulated by law according to the signalling and automatic train protection (ATP) system provided. Trains are only allowed to run at 80 mph and over if ATP is provided in some form. Trains are restricted to below 59 mph (passenger) and below 49 mph (freight) in "dark territory", i.e where there are no signals. See also US Railroad Signalling.
There will often be circumstances where one train will be required to follow another in the same direction. Without signals, this can be hazardous unless certain rules are followed. In the UK, single line sections are comparatively short and are protected at each end by signalling but, in the US, they can be very long and without signals. In the US, trains following one another over an unsignalled single line are only separated by time interval.
The normal time interval is five minutes between trains dispatched from a station. If the train fails to keep to the normal speed for that section, the conductor or "flagman" will drop a flare (called a fusee) with a five minute burn time. The following train is not allowed to pass this flare while it is still alight. If his train stops, the rear conductor has to protect the train by walking back a safe braking distance and laying detonators (called torpedoes in the US) and showing a red flag or light to following trains. This is similar to the emergency protection used in the UK.
Time interval operation is not used in isolation. The basic form of authority for single line operation is the Train Order. This is a written instruction passed to the train crew which tells them they may pass onto the specified single line section and proceed along it until its end or a loop or siding is reached. Once there, the Train Order will indicate whether they must wait for a new order or wait until a train running in the opposite direction has passed before they can proceed further. Train Orders have formed the basis for train movement control in North America since the 1850s.
Train Orders were traditionally always issued as a written paper which was handed to the crew. Dispatchers were in charge of issuing TOs since they has the overall picture of the section of line under their control. Communications between dispatchers were originally by electric telegraph using Morse code. Later, telephones became the standard method. If orders were changed along the line, a train could be stopped at a station by a manually operated fixed signal. This signal was only an indication to the driver that he should stop and collect a new TO.
In recent times, radio has become the standard method of issuing train orders. Instructions are passed to crews by radio and the driver copies his instructions onto a special sheet called a Track Warrant.
Fig. 2: Sample Track Warrant form (BNSF).
The driver will repeat back the message to the dispatcher to ensure the message has been properly understood. This elaborate system is essential to ensure that the correct messages are received and understood by the designated crew. A summary of how the system works is as follows.
Unsignalled Operations in the US
The system used on most unsignalled lines in the US is called the Dispatcher Control System or DCS. An unsignalled line is broken up into blocks which can be up to 20 miles long. Each block has a Block Limit Station (BLS) at each end. A BLS consists of a simple fixed sign with the name of the BLS on it. Block Limit Stations are usually located away from sidings so that any shunting can be done within the limits of a single block.
Train operation is controlled by a dispatcher. The dispatcher will issue a track warrant called a "Form D" (on many lines, some call it by other names) to the train over the radio. This will be his authority to proceed. The Form D tells the train which blocks it can be in.
If a "meet" is set up (i.e. trains have to pass each other) the dispatcher will order one train into a loop (called a siding in the US) and, once the crew calls in via radio that they are clear of the main line and hence out of the block, he will issue a Form D to the other train.
If another train needs to follow the first, the dispatcher may instruct the first train to call in via radio each time it passes a BLS. In this way the dispatcher knows which blocks are occupied and, therefore, where trains can run safely. To keep the trains a safe distance apart, the dispatcher will either use time delay at each BLS or will issue a Form D to the lead train and a separate Form D to the following train at each BLS.
Sometimes, lines which have signalling need to use the Dispatcher Controlled System (DCS), for example, wrong way running being required on single direction, automatically signalled tracks.
An alternative description of the use of track warrants is at a useful page called Track Warrant Control.
Centralised Traffic Control (CTC)
The more heavily used single lines are nowadays signalled and remotely controlled from a central location. Track Warrants are largely unnecessary. The signalling is based on track circuits and often allows long single line sections to be broken up into blocks, each protected by a signal at each end. More than one train can proceed in the same direction, fully protected by automatic signalling.
Single Line Operation (UK)
In the UK, single line sections were generally short and were normally controlled by signalmen. Under manual operation, trains were admitted to the single line once the signalman at its entrance had confirmed with the signalman at the exit that the previous train had cleared the other end. Various systems were used and many can still be seen on local or preserved lines today.
Staff and Ticket System
The original process for single line operation was known as the "staff and ticket" system. Authority to enter the single line was by the signalman at the entrance giving the driver a "staff", a rod of wood or metal, on which the name of the single line was stamped. When the train arrived at the other end of the single line, the staff was given up to allow a train to proceed in the opposite direction.
If more than one train was required to follow consecutively in the same direction, the first and subsequent trains were given a "ticket", which was analogous to the US train order. It detailed the train information and the section over which it was allowed to pass. When the ticket was issued, the staff was also shown to each driver as a guarantee that the issue was OK. The staff was carried by the last train of the batch going in that direction.
Electric Token Block
From the late 1880s, various forms of electrically interlocked, single line token systems were introduced. The signal box at each end of the section was equipped with a "token instrument". This was a machine which detected the removal or replacement of a "staff" or "token". The token was a metal key which was smaller than a staff but which performed the same function. The token instruments at each end of a single line section were electrically interlocked so that the act of removing a token locked both machines and prevented further removals from either instrument until the missing token was replaced at one end or the other. To allow the passage of two or more consecutive trains in the same direction, the system used more than one token. In effect, the "staff" and "tickets" now became a set of metal tokens. In some systems, a set of "staffs" were used as tokens.
It worked like this: The signalbox at each end of the line had lots of tokens. They were stored in the token instrument. A token for authority for passage along a single line could only be removed by the signalman at the departure end if the signalman at the receiving end pressed a plunger which released the token on the departure end instrument. When the token was removed from the instrument at the departure end, the instruments at both ends became locked. Both remained locked until the token was replaced in the receiving end instrument. If a second train had to follow the first, a second token was released by the receiving end signalman as before and removed from the departure end, locking both instruments again until it was inserted in the instrument at the other end. Because it was a system relying on electric locking it was called the electric token system.
The next evolution was called "tokenless block" working, where the signals at each end of the single line were themselves electrically interlocked. The signalman could not clear any signals protecting the entrance to the single line as long as the signals at the other end had admitted a train. A direction lever was provided at each end and they had to match each other to enforce the locking.
Radio Electronic Token Block (RETB)
The development of modern electronics and "vital" or secure radio transmission systems has allowed railways to develop more cost effective signalling. In rural areas of the UK, where long sections of single line require token block operation, a system for centralised control, using modern computer technology, was adopted. It is known as Radio Electronic Token Block (RETB).
Fig.3: Diagram of route with Radio Electronic Token Block system
At a number of locations in the UK, this system (Fig.3, above) has been in operation for over 10 years and allows one signalman to control several single line sections between passing loops. One installation in Scotland controls over 100 miles of railway.
Each train operating over the single line is equipped with a special speech and data radio transmitter/receiver with a unique identity. At the start of the single line, the driver stops and calls the control centre for authority to enter the section. If the line is clear, the signalman in the control centre transmits a coded "electronic token" data message which is received by the train and then shows the authority for that section on a cab display. The driver will then call for confirmation that he can enter the section. Once in the single line section, he will advise the control centre that he has cleared the loop track. A clearance marker board is specially provided to help him.When he has reached the end of the single line section, the driver calls the control centre again and offers to give up the token. After a "handshake" procedure by the control centre, he sends the token back by radio data transmission to release the section.
The signalman is provided with a computer controlled radio system which allocates the coded tokens to each section and prevents more than one token being issued for an occupied section. It also receives the tokens sent back by each train as it reaches the end of the single line section. At the exits of the single line sections, the points are permanently set in the direction of normal running and are "trailable" for trains entering the section, i.e. they allow a train to pass through at reduced speed using the wheel flanges to move the point (switch) blades aside reset to the normal position.
A "Distant Board" complete with AWS ramp, warns the driver that he must slow down for the movement over the points leaving the single line. The Points Indicator shows the position of the points. A "Stop Board" at the end of the passing loop warns the driver to stop and ask for permission to enter the next single line section. The "Loop Clear" board shows when the rear of the train is clear of the points.
An excellent description of the operation of RETB is available at The Modernisation of the Cambrian Lines by Alan Jones on the Signal Box Site. Of course, this system is now being superceded by the test installation of ERTMS on the route.
Additional information on US single line operations supplied by "Evil Mike".
More articles on signalling:
The Development and Principles of UK Signalling - Metro Signalling and ATP - ATP Transmission and Moving Block Systems - ATO - Route Signalling - ATC - UK Warning & Protection Systems - UK Signalling - What the driver sees - US Signalling