I hope West Wales services continue after the electrification somehow.
It'd make sense for either ATW crew the morning Carmarthen-London as far as Swansea or FGW taxi them out there/stay overnight in Carmarthen or even have a few Carmarthen based crews. The best way of operating would be if there was an ATW service to take crew between Swansea and Carmarthen.
Training lots of ATW staff to operate a train type that works west of Swansea twice a day. Putting up FGW staff who live in Swansea in a hotel in Carmarthen. Basing FGW crew in Carmarthen to work one train out and one train in a day. Leaving a train parked overnight about 45 minutes' journey time from a TMD (bit different from sets left at Hereford and Worcester). Not my idea of sense.
There already are ATW services between Swansea and Carmarthen before the morning train leaves and after the return service arrives but if a train needs fuel, water, inspections, running repairs etc, why would you not send it back to the depot anyway?