Single-line working
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On a double track railway, single-line working refers to the practice of using one track out of two, usually when one of the tracks is out of use for maintenance or because of damage or some obstruction.
[edit] Circumstances
Tracks that are signalled for bi-directional operation can be operated as a single line without any special measures, since the signalling equipment is already in place to permit trains to run in either direction over the single track. However, when a section of track which is not normally bi-directional has to be used in both directions, very special safety measures are needed to ensure safe operation. On a single line the worst safety risk is a head-on collision between two trains travelling in opposite directions along the single track, but there is also the risk of a rear-end collision if two trains travel in the same direction, and the first train stops for some reason. Because the normal signalling system is not in operation, either of these two risks must be carefully guarded against by the system of operation used.
[edit] Token working
A common solution to the problem is to implement some form of token working. If the situation is a planned one, a normal token may be specially provided, complete with tickets. Alternatively, a pilotman might be provided.
A pilotman travels on trains which traverse the section of single track. Provided there is only one pilotman the system is safe — the pilotman acts as a human token authorising a driver to proceed over the single track. However, a complication arises with this system if two trains are to traverse the single track in the same direction: the pilotman can travel on the first train, but then may have to walk back along the track in order to accompany the second train.
A solution to this is to provide a locomotive for the pilotman to travel on; this locomotive can be attached to the front of any train that is to pass through the single-line section, and the locomotive can return 'light engine' in order to convey the pilotman back through the section in order to pilot a second train.
Alternatively, the pilotman authorises the driver to take his train through the section, but instead of travelling on the train, he remains where he is in order to authorise or travel on the next train to pass in the same direction (this is the same principle used for "staff and ticket" working). He then waits until he has received a message from the signalman at the far end of the section, confirming that the train has arrived complete. He can then authorise or travel on the next train to travel through the section.
[edit] Further reading
Rolt, L. T. C. (1986) Red for Danger, a history of railway accidents and railway safety, London: Pan Books. ISBN 0-330-29189-0 (4th edition revised and with additional material by Geoffrey Kichenside.) (Chapter 6: single-line collisions)
