Mechanical railway signalling installations rely on lever frames for their operation to interlock the signals, track locks and points to allow the safe operation of trains in the area the signals control. Usually located in the signal box, the levers are operated either by the signalman or the pointsman.
The world's largest lever frame (191 levers) is thought to have been in the Spencer Street No.1 signal box in Melbourne, Australia, which was decommissioned in 2008. The largest operational lever frame, meanwhile, is located at Severn Bridge Junction in Shrewsbury, England, and it contains 180 levers, although most of them have been taken out of use.
The lever frame is located in the signal box, which can be a building at ground level or a tower, seperated from or connected to an existing station building. Early lever frames were also built as ground frames next to the track, without any form of shelter and were usually operated by traincrew and not permanently staffed. Especially in England, lever frames with the pivot underneath the floor of the signal box were common (as seen on the photo on the left). This design's relatively short lever angle is a major disadvantage, as it requires more force to move the lever. Therefore, later, especially in Germany, lever frames with pivots inside the signaller's room were used, that allow for a lever angle of approximately 180° (as seen on the photo on the right).
By the movement of individual levers (or sometimes cranks), signals, points, track locks, level crossing gates or barriers and sometimes movable bridges over waterways are operated via wires and rods. The signaller chooses the correct combination of points, facing point locks and signals to operate, which will control the movement of each train through their area of control. The lever frame contains interlocking designed to ensure that the levers cannot be operated to create a conflicting train movement. Each interlocking installation is individual and unique to the location controlled. The interlocking may be achieved mechanically or by electric lever locks, or (more usually) a combination of both.
Signals or points located some distance away from the signal box are sometimes electrically rather than mechanically operated. Movement of the controlling lever operates an electrical circuit controller. In the UK, it is practice to cut short the handles of any levers controlling electrical apparatus, to remind signalmen that little effort is required to move them. Mechanical lever frames and interlocking have now largely been replaced by modern, much larger electrical or electronic route interlockings located in Power Signal Boxes and more recently Integrated Electronic Control Centres which are able to control much larger areas of the rail network.
Mechanical lever frames
The lever frame holds individual levers, which are usually sorted in groups depending on their type, mounted on discs. The distance between the levers is approxiamtely 140–160 mm. Behind the levers, a box that houses the interlocking apparatus is mounted on the lever frame. For better maintenance, the lid of this box is made of glass. To assist the operator in determining their functions, each lever in a frame will generally be uniquely labelled, one common method being to number the levers in order from left to right. A lever's identification may be painted on its side or engraved on a badge or plate fitted either to the lever or behind it. This may be accompanied by a description of the lever's function. Usually, a large track diagram is positioned within easy view of the operator, which clearly shows each lever number adjacent to symbols representing the items of equipment that they operate. Levers are commonly coloured according to the type of equipment they control, the code of colours varying between different railway administrations. For example, in British practice, the following code generally applies: a red lever controls a stop signal or shunt signal, a yellow lever controls a distant signal, a black lever controls a set of points, a blue lever controls a facing point lock, and a white lever is spare. Brown levers are used to lock level crossing gates. In Germany, signal levers are red, whilst levers for points and track locks are usually blue, and route lock levers are green. Also, individual numbers and letters are used to indicate each individual item a lever operates in Germany as well.
There are several different types of power frames: hydraulic, pneumatic, electric, electro-pneumatic and pneumatic with hydraulic actuation. The lever frame principle of operation was perpetuated in the earliest power signalling installations.
In hydraulic lever frames, moving the lever operates a hydraulic valve rather than wires and rods. To prevent accidents, operating a set of points requires pulling the actual lever for the points and a secondary check lever. The points are then moved by a hydraulic motor. This type of power frame has the disadvantage of a relatively low distance between points and signal box (approximately 200–250 m) and a slow operating speed. It was common in Italy and France only. Pneumatic lever frames have an operating principle that is related to that of hydraulic lever frames, however, instead of a hydraulic liquid, compressed air is used. The two types also share the same disadvantages. Pneumatic lever frames were common in the United States. Later, they were replaced with electro-pneumatic systems, where the valves were no longer operated with levers, but electric switches instead. This allowed for a greater distance between signal box and points. Whilst first being common in the United States only, this system was also used in the United Kingdom later. In Austria, Siemens & Halske built a fully electric power frame in 1894, which does not rely on compressed air. Instead, electric motors move the points. Later, this system was also used in Germany. Since the signals and points worked electrically, no mechanical effort was needed to move the levers, which could therefore be miniaturised. In some cases, the interlocking was still done mechanically, but in others, electric lever locks were used. Westinghouse Brake & Saxby Signal Co. Ltd. was one of the companies that produced miniature power lever frames. All-electric power lever frames were produced from 1929 onwards; earlier mechanical frames were produced before this date. In the UK, the Style 'L' all-electric miniature power lever frame was the most predominant on UK main line railways. The Style 'B' & 'K' miniature power lever frames were the historical precursor frames in the development process of the Style 'L' frame. London Underground railways used miniature power lever frames; initially Style 'B' was used. Later on, Style 'N' was introduced. This is a mechanically locked version of the Style 'L' frame. Style 'M', 'M2' and 'V' were also used.
In France, pneumatic systems with hydraulic actuation were used, where the points and signals were moved by hydraulically actuated pneumatic motors. Signal boxes with such a design were operated with route lock knobs only. Moving a route lock knob would automatically choose the correct combination of points and signals to set a certain route. The signaller then just had to confirm the route to operate the signal. Moving back the route lock knob to the default position does not have any effect upon points. The points would only be moved by the following operation of a different route lock knob. Whilst being easy to operate under normal conditions, managing impairments and shunting is considerably difficult with this type of power frame system.
In the UK, larger railway companies like the Great Western Railway (GWR) and the London and North Western Railway (LNWR) developed their own mechanical interlocking systems, whilst most smaller independent railway companies installed signalling products and systems bought from such firms as The Railway Signal Company (RSC) and The Westinghouse Brake and Signal Company (WB&SCo).
- This list contains manufacturers of lever frames
- Aster (France)
- Bianchi-Servettaz (Italy)
- Fiebrandt & Co (Germany)
- Max Jüdel & Co (Germany)
- The Railway Signal Company (United Kingdom)
- Scheidt & Bachmann (Germany)
- Siemens & Halske (Germany)
- Stahmer (Germany)
- Vereinigte Eisenbahn-Signalwerke (Germany)
- The Westinghouse Brake and Signal Company (United States and United Kingdom)
Inside a signal box on the Isle of Wight Steam Railway
McKenzie & Holland lever frame at Eastgates signal box, Colchester, UK
Derby Road Signal box, Ipswich, UK, showing the McKenzie & Holland frame, taken in 1997. Opened in 1891 by the Great Eastern Railway. Abolished and demolished in 1999.
- Wilhelm Adolf Eduard Cauer: Sicherungsanlagen im Eisenbahnbetriebe, in Handbibliothek für Bauingenieure, published by Robert Otzen, Springer, Berlin/Heidelberg, 1922, ISBN 9783662344903, pp. 122
- Wolfgang Fenner, Peter Naumann, Jochen Trinckauf: Bahnsicherungstechnik: Steuern, Sichern und Überwachen von Fahrwegen und Fahrgeschwindigkeiten im Schienenverkehr, John Wiley & Sons, 2011, ISBN 9783895786839, p. 89
- Hinson, John. "Spencer Street No1 cabin, Melbourne, Australia". signalbox.org. Retrieved 15 September 2018.
- Hinson, John. "Severn Bridge Junction signal box". signalbox.org. Retrieved 15 September 2018.
- Stephen, Paul (July 2018). "FROM THE FILES: Shrewsbury's record-breakers". www.railmagazine.com. Retrieved 20 September 2018.
This place is fairly unique these days in being double-manned, but with 92 levers to operate it keeps us fairly busy and you soon work through the shoe leather.
- Cauer: Sicherungsanlagen im Eisenbahnbetriebe, p. 122
- Cauer: Sicherungsanlagen im Eisenbahnbetriebe, p. 123
- Cauer: Sicherungsanlagen im Eisenbahnbetriebe, p. 125
- "South Devon Railway - Railway Signalling". www.southdevonrailwayassociation.org. South Devon Railway Association. Retrieved 20 September 2018.
- Cauer: Sicherungsanlagen im Eisenbahnbetriebe, p. 126
- Cauer: Sicherungsanlagen im Eisenbahnbetriebe, pp. 250
- Cauer: Sicherungsanlagen im Eisenbahnbetriebe, p. 250
- Cauer: Sicherungsanlagen im Eisenbahnbetriebe, p. 251
- Cauer: Sicherungsanlagen im Eisenbahnbetriebe, p. 252