63 Developing signals A Turner Dec 2017
At our December meeting we were pleased to welcome for the fourth time our speaker Alex Turner, a retired senior civil engineer with British Rail.
On this occasion Alex focused on the development of signalling and its contribution to the safety of rail travel. In the early days a signal box could only control the movement of trains within its own section or block and had no reliable means of knowing when a train was about to enter or leave its block. However the advent of the telegraph system meant that signal boxes were able to communicate by a bell system with the adjacent boxes and so confirm that a train had cleared a particular block before a following train was allowed to enter. The length of the block which could be controlled by a signal box was limited because a signalman could only be expected to have enough strength to pull a reasonable length of wire connecting a signal lever to its semaphore signal or an even shorter length of the metal rod which connected a lever to switch its set of points.
The introduction of electric coloured signal lights and power operated points meant that signal boxes could be situated much further apart and their number reduced. The introduction of track circuiting meant that on sections of track with no points or crossings trains could automatically set a signal at red when they passed it and it would only return to yellow and then green when the train activated the track circuit at a specified distance up the line. Nowadays track circuits can also provide information about the position of trains along a particular route which can be fed into the various computer screens and circuit diagrams situated in the control centres and signal boxes throughout a region. This, and the simplification of track layouts, has resulted in a major closure of signal boxes; the first part of the East Coast mainline for instance is controlled only by the boxes at King’s Cross and Peterborough. Network Rail has plans to further reduce the number of signal boxes or signalling centres on the whole main network to 15 or 17. It is also considering plans to introduce a moving block system whereby information on the position of trains is transmitted to other trains and the system will automatically control the space between trains. Such a system is already In operation on the Docklands Light Railway, but as Alex pointed out it is much easier to install new systems on a brand new railway than it is to adopt them in an existing railway.
Any signalling system itself cannot provide absolutely safety and depends a lot on the correct human response and Alex described a number of examples where human error resulted in serious accidents. As a result of such accidents various systems were introduced to guard against the possibility of drivers passing signals at red. The first was the Automatic Warning System which sounded a warning in the cab if the signal ahead was at red but it relied upon the driver responding and applying the brakes if necessary. This was subsequently replaced throughout the network by the Train Protection Warning System which calculates the speed of the train and if it is too fast for the particular situation automatically applies the train’s brakes. The system can be seen for instance in Huntingdon station and identified by a pair of metal grilles situated in the centre of the track. It should be noted that the introduction of a wireless moving block system will also not only help to reduce human error but also make the railways less prone to vandalism by reducing the amount of copper cabling which is contained in the ducts which run alongside the tracks.