Single line working using tokens
Early railways used slow speeds and had frequent passing places, so it was safe for trains to be driven 'on sight' as far as the driver could see was clear. However as speeds and distances between passing places increased, it was quickly found that more complex systems to avoid collisions were needed on single lines.
At first this was done using a unique object called a 'train staff' for each section of line, which the driver of each train was required to carry with him. As there was only one staff for each section, this ensured only one train could be on the line at once. It worked well if trains always alternated in direction, but if two trains in succession were required to travel in the same direction, the token had to be walked back to the start after the first train. Because of this the 'staff and ticket' system developed: the first train would be shown the staff and given a paper or metal 'ticket' to carry on the engine. The final train, in a group travelling in the same direction, would carry the staff.
With the staff and ticket system, there was little to stop the signalman issuing tickets improperly. Token systems were developed as one way around this problem, using electrically-connected machines called 'token instruments' to hold a set of objects like the earlier train staff, called 'tokens'. The machines were designed to be linked in pairs, with each pair of machines sharing a set of tokens, and designed so that only one token in the set can be removed from the instruments at any time. The first token instruments used metal batons ('electric train staffs') or discs ('tablets'), and tokens always had to be removed from one machine and returned to the other. The SVRSevern Valley RailwaySVRA:Severn Valley Railway AssociationSVRS:Severn Valley Railway Society uses a later type of instrument called a 'key token' instrument, so named because each token resembles a large key. Examples of early tokens and instruments may be seen in Kidderminster Railway Museum.
The token instruments
The largest part of each token instrument is a 'magazine' consisting of four vertical channels, which stores each machine's stock of tokens. Because each token has a large head, wider than each channel, they can be moved around inside the magazine but not removed. Above the magazine is a lock mechanism called a 'commutator'. This is the only route by which tokens may be taken from or placed into the instrument, by turning the token like a key in a lock.
Alongside the commutator is a bell plunger used to signal to the other signalbox, a telegraph needle which moves when the plunger in either instrument is pressed, and an indicator which reminds the signalman whether a token has been released or not. Some token instruments have automatic electrical indicators, but all of the instruments on the SVRSevern Valley RailwaySVRA:Severn Valley Railway AssociationSVRS:Severn Valley Railway Society have manual ones.
Tokens can be placed into an instrument at any time, if they fit, but a token can only be removed if all the tokens are in the two instruments, and the signalman at the other signalbox is holding down the bell plunger on his instrument.
Each instrument is numbered, and each token, like a key, only fits into the correct instrument. The tokens are also stamped with the names of the signalboxes at either end of the section they apply to, and are colour-coded, with all the tokens in a set having their handles painted a distinctive colour.
When a token has been removed from an instrument, it is usually placed in a 'token carrier', a metal holder which is about the same size and shape as an unstrung tennis racket. This enables the tokens to be passed to and from moving trains.
The signalman sending a train will alert the receiving signalman with the "Call Attention" bell signal, and when he or she responds will, by bell signal, ask "Is the line clear for [this kind of train]", there being different signals for different train types. If the receiving signalman accepts the train, he or she will repeat the signal back, keeping the bell plunger held down on the final beat of the signal. Holding down the plunger releases the mechanism of the sending signalman's token instrument, so he or she can remove a token. When the signalman removes the token both signalmen will set their indicators to show that a token is out, and which direction it is travelling in.
Removing the token will unlock the signal which allows trains to enter the section of line between the two signalboxes. The signalman will clear this signal, put the token into a token carrier, and give it to the footplate crew, who will read the signal box names on the token out loud so all of the crew know they have the right token for their journey. As the train departs, the sending signalman will check it is showing the correct tail lamp at the end of the train, and send the "Train Entering Section" bell signal to the other signalman.
When the train arrives at the second signal box the crew will hand the token, in its carrier, to the signalman there. When he or she has seen the tail lamp on the back of the train, to prove the whole train has cleared the section, he or she will place the token in their instrument, put their indicator to normal, and send the "Train Out Of Section" bell signal to the sending signalbox.
When two trains are passing each other at a signalbox, the sequence of operations is slightly more complex. This is because only one train is allowed to enter the crossing loop at once, to minimise any risk in the rare chance that either train overruns any signal.
The signalman at the crossing place will keep all his or her signals at Danger until the first train arrives and stops outside the station. He will then clear the signal for that train - let's call it Train A - to enter the station, and collect the token from it as it arrives.
When trains are crossing at a station, the signalman needs to be able to put the tokens back in the instruments before he has been able to see the tail lamp on the back of the train. Because of this, when a train arrives at a crossing station, the train's guard will tell the signalman that the train has arrived complete with tail lamp. When the guard of Train A has done this, the signalman can put the token he has just collected into the token instrument. He will then immediately ask to take it out of the instrument again, for Train B to go back in the opposite direction. If Train B has arrived, it will have been kept waiting outside the station. Once the signalman has the token he needs to give to Train B, he will clear all Train B's signals then go and exchange tokens with Train B, collecting a token from the traincrew with one hand and handing the new token over with the other.
Once Train B's guard has told the signalman that his train has also arrived complete with its tail lamp, the signalman can repeat the token procedure with the token he has just received from Train B, again immediately requesting to take it out again to give to Train A. When he has taken it out, he will clear Train A's signal to depart and walk down to the engine to hand the token over to the traincrew. By the time this has been done, Train B will probably be ready to leave, if not actually pulling out of the station. When the signalman returns to the signalbox he will send the "Train Entering Section" bell signals for Train B, and for Train A as it leaves. As before, he still has to check that both trains are showing the correct tail lamp on the end.
Because of this procedure, it is normal that the first train to arrive at a crossing place is usually the last one to leave it. The timetable will be written to take account of this. The timetable is also normally written to try to make sure that the first train to arrive will be the one on the furthest track from the signalbox. This makes things much easier for the signalman and helps the crossing go more smoothly, because the signalman will have less walking to do.
Crossing trains in this way is only done at two stations on the SVRSevern Valley RailwaySVRA:Severn Valley Railway AssociationSVRS:Severn Valley Railway Society: Arley and Hampton Loade. Although trains can also pass each other at Bewdley, this is not technically 'crossing trains' as described here, because Bewdley station has a stretch of double track between its two signalboxes, signalled using the Absolute Block regulations.
If the token instruments are not working properly, so that tokens cannot be taken out of the instruments, the safety of trains is maintained by appointing a member of staff as a 'Pilotman'. He takes the place of the tokens, by travelling back and forth on the footplate of each train according to the signalmen's instructions. Because tokens are not being withdrawn from the token instruments the section signals remain locked at Danger, so the Pilotman, under the signalman's instructions, tells the traincrew when it is safe to pass these signals at Danger.
The Pilotman can be identified by a red armband that he must wear at all times. Strictly speaking, Pilotman working is closer to the 'staff and ticket' working described above that to token working. If more than one train will be travelling in the same direction, the Pilotman will write out a 'Pilotman's ticket' for the first train(s) to carry, and will travel on the last one.