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Anchor's GSC wasn't superseded by the Transit network, I worked in Birmingham Transit Trunk Test, but it did run along side of it. Digital switching was the death of the Group and Main switching centres. The Transit network (known as Main Switching Centres) was to provide a zero loss transmission network to allow better interconnection of the switching network. The transmission network between Group Switching Centres had a 3dB loss and if you had to go through more than two centres the losses started to make it difficult to talk. So rather than increase the number of routes, which could have been very expensive in building, cable, repeater and exchange costs, a new system consisting of a few new exchanges and using existing routes, buildings and repeater stations was built. If a call had to go through more than two GSC's it was routed through the Transit system instead.
By the way the reason for the losses was that the GSC's were a two wire switching system and there was a 3dB loss across the 2 to 4 wire conversion equipment and the Transit system was a 4 wire switching system. The transmission network was mainly a four wire system to allow amplification.
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