Lever frame
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Mechanical railway signalling installations rely on lever frames for their operation to interlock the signals and points to allow the safe operation of trains in the area the signals control. Located in the signal box, the levers are operated by the signaller.
By the movement of individual levers, signals, points, level crossing gates or barriers and sometimes movable bridges over waterways are operated via wires and rods. The most distant signals or points from the signal box are sometimes electrically rather than mechanically operated upon the movement of its lever. The signaller chooses the correct combination of points, facing point locks and signals to operate, which will control the movement of each train through his/her area of control.
Mechanical lever frames can be used to control colour light signals and power-worked points. 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.
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.
In the UK, larger railway companies developed their own mechanical interlocking systems e.g. the Great Western Railway (GWR) and the London and North Western Railway (LNWR), whilst smaller independent railway companies sometimes installed commercially obtained signalling products from companies such as The Railway Signal Company (RSC) and The Westinghouse Brake and Signal Company (WB&SCo).
The mechanical lever frame is interconnected with the electrical block equipment which the signaller uses to accept trains into his/her controlled sections of line and to offer on to the next block area on in the direction of the train's travel.
Mechanical lever frames and interlocking have now largely been replaced by modern, much larger electrical or electronic route interlockings located in Power Signal Boxes and more recently Integrated Electronic Control Centres which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.
