C O N T E X T 1 2 4 : M A Y 2 0 1 2 31 Andy Savage A future for historic signal boxes What do you do with a set of heritage assets when the whole type reaches the end of its life? The question faces Network Rail over its remaining mechanical signal boxes. The classic mechanical signal box was developed in the 1870s and 1880s in response to a rising number of collisions between trains, as the network became busier, with more and faster trains.Along with automatic brakes, the introduction of mechanical block signalling led to the reversal of the trend of more rail accidents, and started the improvement in railway safety that has continued ever since. Mechanical signal boxes were in many ways forerunners of today’s networked computers, with logic systems that communicated with adjacent systems in a network. However, the logic systems were achieved by mechanical systems of bars and tappets, communications were bell codes, and the system output was the changing of points and signals in a safe order, achieved by the brute strength of the operator. Even with levers to help, this meant that it was possible to control points only over a couple of hundred metres, and signals over a distance of up to about a kilometre. As a result there were a large number of signal boxes – probably over 10,000 at the peak. The technology of the mechanical signal box remains almost unchanged from the 1870s. The introduction of local track circuits has enabled the signaller to see more clearly where trains are, and helped make the interlocking safer, but little else has changed. However, particularly since the second world war we have seen a new type of signal box develop, with electrically driven points and colour light signals. This change took away the need for physical strength from the signaller, and allowed boxes to cover much larger areas. As the same time the introduction of continuous track circuits, and electrical logic and communications, mainly relay driven at first, has mean that the span of control of a single box has grown rapidly. Combined with the closure of rural lines, this has led to a rapid decline in the number of mechanical boxes. There were just over 5,000 in 1960, and just under 1,000 boxes (of all types) by privatisation in 1994. The introduction of solid-state electronics in the 1980s meant that the rate of decline was maintained, although the number of boxes was falling fast. Railtrack, for whatever reason, stopped BR’s closure programme. If this had not happened and the former BR rate of closures had continued, the classic signal box would have been extinct by the millennium. However, Network Rail has now picked up the challenge. It proposes to concentrate all its signalling and train control into 14 national centres over the next 20 years or so. There are now some 800 signal boxes in the UK, of which about 500 are mechanical. The smallest boxes, which include almost all the mechanical ones, are Celebrating the presentation of an award to the St Albans Signal Box Preservation Trust
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