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Railway Signalling |
All the systems so far described rely on the integrity and vigilance of the signalman, Some form of aid was clearly needed to help the 'Bobby in the Box'. The earliest form of train detection was the simple treadle. This is a simple pivoted metal ramp that is mounted on the inside of the running rail. When a train passes over the treadle the flanges of the wheels depress the ramp on its pivot operating an electrical contact, in most cases these are now only used where track circuits are unreliable such as platform ends where the track is likely to be dirty or rusty.
 Treadle at rest (no train) |
 Treadle operated (Train over treadle) |
Another use of the treadle is as a fouling bar to prevent the signalman from changing a set of facing points while a train was standing on them. the treadle is connected to a locking bolt which engages the tie bar across the heal of the points.
 Facing point lock fouling bar at Goathland |
 Locking Bolt at Grosmont |
 Facing point lock outside Carriage & Waggon at Pickering |
 Carriage & Waggon Ground Frame. Pickering |
The locking bolt and the treadle are connected to a lever (usually blue) in the signal box. All the signals associated to the set of points are interlocked so they cannot be changed unless the bolt is engaged. If a train stops on the points, then the signalman would not be able to withdraw the bolt because the treadle cannot rise because of the wheel flanges,
On the ground frame the FPL lever is the one on the far right of the frame. This lever is locked remotely by the signalbox when no shunting is taking place. Along side the frame is a telephone to talk to the signalman at New Bridge.
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