Geographic[al] interlockings are pre-wired, pre-configured, and pre-tested modules of relays that perform a specific function like a controlled signal, or a point end, or an overlap end point for example. Each module is a sealed box of a whole bunch of relays - sizes vary but let's say 1ft wide by 2ft tall by 8" deep maybe. At the back/top of each box are a number of multi-pin couplers - a point end would have three connections, for example, plus a pair of ring cables. Multi-core cables then connect modules together, laid out just like the track layout. Theoretically you then have a working interlocking very quickly. Customisation is possible with certain contacts and bridging terminals exposed to the outside of the module. These would mostly be for approach control and approach locking, though less needed in later variants as even this was built in to Westpac IV at least.
However, the nature of the pre-configured modules means that there was a lot of redundancy in either relays or relay contacts as the module was designed to support all features, while not all might be used. So the per-product cost could be high, but installation/testing cost low.
Westinghouse produced at least 4 variants (or improvements on the previous version); GEC at least two. There have been no new major installations this century as far as is known, as SSI is cheaper to produce, install, and maintain.
Last edited by GeoffM on 15/09/2016 at 03:01