But why straight diesel ?. Perhaps the PROPORTION of the Cheltenham route that is electrified is the factor - too little ?. Maybe the routes the 756s are meant for are more likely to get electrified. So instead of wondering why the 231s are straight diesel the optimist in me prefers to wonder why the 756s can use electrification.
That's highly likely why the decision was made. Cheltenham to Maesteg is about 90 miles but only about 20-25 miles are actually electrified, so it'd spend less than a third of the journey on the juice. Unfortunately it doesn’t look as if Cardiff-Bridgend-Maesteg and Severn Tunnel Jct-Cheltenham will be electrified any time soon, and there are no other OHLE routes in Wales as yet to take advantage of.
How about performance? From Stadler’s data sheet, the 231s look very much like 755s but without the several tonnes of transformers and overhead line equipment, and probably with lower motor gearing too (top speed is 90mph to the 755’s 100mph). So it should have superior performance to a 755 on diesel which was already faster off the line than most 2nd-gen EMUs! So 231s should have no trouble keeping up with the heavy 800/802s on the 90mph electrified section.
The Stadler FLIRT is a modular design so converting the units to bi-mode when the need arises shouldn’t be an issue. Maybe they are cheap out, but if they're a bit faster and cheaper to run and maintain that's a plus
Thanks for the vids and pics everyone, excited to see these finally in Wales!