Not going to wade through all the pages of this thread (not right now anyway), so this may have already been said, but:
Carmarthen - Cardiff.
Rail Journey Time: most take 1hr 40mins or more, but one does it in 1hr 33mins
Arriva Buses Wales 20 service: 1hr 26mins (ok, it's meant to be a coach service, not a bus, but Arriva seem to use Wright Pulsars on it, not sure how often that is compared to use of the coaches though)
The 20 only runs twice a day, but it shows how the M4 completely trashes any hope of rail being time-competitive for Carmarthen - Cardiff via Swansea. If express rail services using the Swansea District Line were introduced (calling at Llanelli, Port Talbot Parkway and Bridgend only) they could do it in arround 1hr 23mins (going by the diversionary timetable showing Cardiff - Llanelli at 1hr 1min and the current 1 train per day that doesn't stop at Pembrey and Burry Port doing Carmarthen - Llanelli in 22mins).