This is a rather magnificent device. When a release is given and the switch release plunger is pressed the switch jumps to the mid position. The signalman then turns the switch to 'trolley on line' and then press token release to release the Yale lock on the trolley token...

This was the first type of AHB crossing indicator. I can usually work out how connect instruments correctly but this one threw me for a while. I did a 'best guess' and had it working on the shelf but i knew it wasn't right.

An internal shot showing the post office 3000 type relay and six way terminal block.

This is the diagram for the internal wiring of the barrier inication unit.

This is the diagram for the external wiring of the barrier inication unit. During the time i was trying to work out how the unit worked i never would of guessed t3 needed a permanant feed to, in effect, turn POR into a polar relay

The two relays at the signalbox end. The picture was taken just after installation. The wiring is a little neater now.....

Where on earth was this instrument used? This has really got me thinking. It provided two full evenings of fun making a correlation sketch and checking it all worked. Pictures of the plunging units are posted with the owners consent..