The RETB Story
In the early 1980s a heavy snow storm brought down a large amount of lineside wires used for telephone and signalling north of Tain. This caused great inconvenience and the problem was overcome by the research and development department getting the signalling instruments to work by radio instead of wire. They then went on further to develop a system whereby the whole line from Inverness to Wick/Thurso and to Kyle could all be done from one location using only one signalman.
A radio was fitted in the cab able to receive and transmit and connected to what is a cab display unit (CDU) with a send/receive button and an interlocking key to be used to work ground frames to enter sidings.
As there are no singalmen at stations with crossing loops to shift the points manually, they were converted to hydaulic operation. These are held by hydraulic pressure to the position for a train to enter the left side of a loop (except Muir of Ord). A train leaving a loop merely squeezes its way through the points. An amber light called a points set indicator (PSI) lets the driver approaching the points know that they are in the correct position and he proceeds through the points at a maximum speed of 15mph. If the PSI is unlit he must inform the signalman, who instructs him to clamp and scotch the points. He then takes the train over the points, stops, and removes the clamp and scotch (a wedge) and replaces them in the lineside cabinet. This procedure takes appoximately 15 minutes, so if you get two sets of points out of position on a journey the train is then half an hour late. Power cuts in winter can cause problems to the PSIs and point heaters and if the cabinet locks are also frozen then further delays are caused. This could be saved by having the clamp and scotch on board the train.
Stop boards replaced the signals and a driver has a token displayed on the screen of his CDU. He also has the signalman's verbal permission to pass a stop board. Other boards are loop clear marker boards.
The radio network consists of a chain of radio transmitter/receivers called base stations. They are similar to mobile phone masts, with the radius of signal being approximately 30 miles. In between each base station is a re-broadcaster or what is better known as a repeater. Each base station is on a different channel, requiring the driver to change channel for the area he is in. For quite a lot of the time it is possible to get two channels, which can be quite helpful if one channel is not working correctly.
The signalbox equipment consists of a dedicated keyboard to input the required information and four video screen monitors which display a diagram of the track from Inverness to Wick/Thurso and to Kyle. In the equipment room is the interlocking rack consisting of modules which operate by "majority voting" where two out of three must agree that what is going from the keyboard to be processed is correct. This rack is the same system as is used at most major signalling centres and is very reliable. The radio network is an overt system in that everyone can hear all transmissions. As well as trains receiving tokens which permit only one train in a section at a time (e.g. Muir of Ord to Dingwall), engineers can also get a token so that they can do track work without the lookouts or flagmen, the token being the means of protection.
When the train arrives at Georgemas from Thurso the driver returns the token, then walks through the train to the other end. The cab display unit number has to be changed to the one he is then to use, this takes 30 seconds and a token is taken to travel to Forsinard. At all loops the trains have to approach the PSI being prepared to stop. The train must be at a stand to exchange tokens (although it can be done on the move but the rules and regulations dictate "at a stand" and this can be checked through the black box). If a front radio fails the rear one is used, which means that the driver has to walk to the rear cab at each token exchange. Long section tokens are not permitted in these circumstances.
A system of dial-ups can be used if a break in the network occurs, allowing the radio system to keep the affected part working. Now the benefits of this RETB system is that fewer signalmen were required; one signalling the whole line north and west of Inverness at one time. However, because of the large amount of level crossings, there is now one other person on duty to answer phones etc. Another benefit is that drivers and signalmen have constant contact, which is very useful for any emergency or if police assistance is required for a train.
If part of the network fails the remainder can still keep working; it is extremely rare for the complete network to fail, it usually requires something in the signalbox to fail for that to happen. The weakest part of the system is usually the repeaters which take the signal on one channel and send it to the next base station on another channel - they have to do this in both directions. The lights now use LCDs instead of a single bulb which was prone to failure but power cuts can still cause them to be extinguished. The cost of renting radio base station and repeater sites is high and can often be difficult to access in winter. There is a lot less flexibility using hydraulic points than operating them conventionally. Dingwall and Georgemas Junctions both involve the driver leaving the cab to operate plungers to power the movement of the points.
It's a good system, but needs good maintenance to get the best from it. If off-the-shelf parts could be used instead of custom made equipment the RETB could serve the north lines for many years to come.
Presently the main problem with the RETB is overload in the Invernet area. RETB was designed for low density traffic and at present its capacity with all transmissions and token exchanges going through one signalling desk is far exceeded, unlike Banavie on the West Highland which has two signalling desks and only around 180 miles compared to Inverness's 230 miles.
The answer to this is TCB signalling Inverness to Lairg (the Invernet area) which I think could be achieved by renting BT lines on a permanent basis. The two main advantages in this are (1) the driver does not participate in the signalling (only to obey the signals displayed) and telephone the signal box if required, and (2) the motive power does not need to be dedicated for RETB. Finally there is definitely no reason that the RETB could not be retained for use Lairg to Wick/Thurso and Dingwall/Kyle.