According to something on another forum, the balises aren't route-specific so anything departing westwards from Central gets an instruction to go into diesel mode whether it's reversing in the electrified Brickyard siding, reversing somewhere unelectrified or going through to Swansea. It may even get that instruction on arrival in the platform, in which case even trains turning round in the platform would start their diesel engines. If true this strikes me as a major design drop-off by someone, considering that at least half of the trains will complete their workings under the wires and by remaining in electric mode would reduce energy consumption and city centre pollution. It may be that the abrupt truncation of the electrification at Central didn't allow any scope to do anything better.
