|Single Track Working|
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.
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, 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.
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. Where a running line is closed (say for emergency engineering work or a derailment), it may be necessary to hastily arrange to operate all trains in both directions over the remaining track. This may also happen for planned but short term engineering work. Such temporary working is implemented by appointing a 'pilot man' who acts in the capacity of the token or staff for the (now) single track line.
 A pilotman (Person in Lieu of Token, hence 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 required to traverse the single track in the same direction: the pilotman can give a personal verbal authorisation to the driver to proceed to all but the last train that traverses in a particular direction. The pilotman will actually travel on the last train in that direction.
This is a parallel of the staff and ticket system of working where the pilotman's verbal authorisation takes the place of the ticket and pilotman's physical presence is the staff. Following an unplanned disruption: it has not been unknown for a pilotman to have to walk back to the other end of the section, or even to be conveyed by road, but this is an undesirable practice because the line is effectively closed whilst this occurs. All movements must always be carried out under the authority of the signalmen responsible for that section of line. On lines that are still worked on the absolute block principle, all train movements continue to be communicated using the block telegraph as though both lines are available.
Another solution sometimes adopted if it is known that there is likely to be service disruption, particularly on long stretches of single track operation, is to provide the pilotman with a personal locomotive (and crew); this locomotive must always be attached to the front of any train that is to pass through the single-line section, and the locomotive can then return 'light engine' in order to convey the pilotman back through the section in order to pilot a second train.
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)
- British Railway Signalling - G M Kichenside & A Williams