FPL Locking Bars - By Andrew Dickinson (Part 4)
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By Andrew Dickinson
Newsletter No 3 refers to the bars on points 44 and 56 as fouling bars. They were however, always known as facing point lockbars (or simply FPL bars) and never fouling bars. Certainly there was other signalling equipment known as a fouling bar. This device used the same rail mounted components as FPL bars but it was an entirely different application. It was provided at some locations where the layout required it at either track convergences or divergences, where it would detect the presence of a vehicle that was foul of the adjacent running line when the signalman attempted to pull the lever that worked the bar.
Also, the term approach locking does not apply to an FPL bar. A facing point is certainly locked by the approach of a train when it is occupying the track circuit over the points or when running over the length of the FPL bar, but this is just a coincidence of terminology. The term ‘approach locking’ relates only to signals applying to facing points equipped with this facility and the ‘holding of the route’ through the lever frame locking when the lever is in the ‘B’ position.
The principal of approach locking is to “hold the route for a period of time (usually 2 minutes) if the signal that has been cleared for that route, with the driver of the train having seen or possibly seen a proceed indication from that signal, is then replaced to danger with the intention of changing the route”. The safety afforded by the provision of approach locking is certainly very necessary if the signalman decided to change the route from a straight run ahead to a goods loop restricted to 10 or 20 mph with the possible disastrous consequences. It was for this reason that approach locking was devised. Approach locking in BR(WR) days had the track circuiting in connection with this, starting at the ATC ramp itself rather than the home signal approach track circuit.
Thanks once again go to Andrew Dickinson for this superb technical information.