Single-tracking
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On a railway, single-line working refers to the practice where when one line out of two is blocked, trains are able to use the other in either direction. This is usually when a line is out of use for maintenance or because of damage or some obstruction It not be confused with Working a single line by pilotman where there is only one line.
Lines that are equipped with bi-directional signalling can operate normally since the signalling equipment is already in place to permit trains to run in either direction over the single line. However, when a section of line which is not normally bi-directional has to be used in both directions, special procedures are needed to ensure safe operation. In single-line working, signals are usually provided for the right-direction of travel but not in the wrong-direction. The worst safety risk is a head-on collision between two trains travelling in opposite directions, but there is also the risk of a rear-end collision if two trains travel simultaneously over the line in the unsignalled direction.
A pilotman will be appointed to take local control of the single line. Liaising with the signaller, he will authorise all movements. Every driver must be given an RT 3193 Single-line working ticket, unless he is going into the single line to:
The Pilotman must travel on every train, unless there is a second train to follow. He must ride with the train if the movement is towards an obstruction or if that is the last train to move in that direction.
A complication arises with this system if the next train is positioned at the other end of the section from the Pilotman. In this instance the pilotman will to have to walk to the other end, or if it is a considerable distance be conveyed by road. This can lead to long delays.
Because signals are provided for travel in this direction, the Pilotman has only to explain to the driver what is taking place. He must state which line is being used for single line working, and between which crossovers it is taking place. If there is an Automatic half-barrier (AHB) crossing under local control on the route, the driver may only pass over it if there is a handsignaller displaying a green flag or lamp to him.
Once he is in possession of the Single line working ticket and has received authority to move from the Pilotman, the driver can obey the signals and may proceed at linespeed if conditions allow.
Since no signals are provided and the train may have to make reversing movements, the driver must be informed of:
The train may have to be authorised to pass a signal at danger in order to begin the movement. When this has been done, the driver may proceed through the single line section at no more than 50 mph.
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Wikipedia