The trouble is the 90mph top speed would not be needed, except on trains to Cardiff, unless we have some linespeed improvments.
I suppose there is the benifit of a uniform fleet there, but if Gareth's suggestion of 12 158s is correct it will not be enough to run the service I proposed above. I estimate:
- 8 diagrams required for Swansea - Carmarthen/Fishguard/Milford/Pembroke (a 155/156 would do for these, if suitably refurbished with decent legroom)
- 5 diagrams required for Cardiff - Morriston - Milford/Carmarthen (these need to be either 158s or 175s)
- Up to 5 diagrams required for HOWL* (a 155/156 would do for these, if suitably refurbished with decent legroom)
* That's a very rough guess for a 2-hourly service extended to include the Crewe - Shrewsbury stopper. Without Crewe-Shrewsbury it would be 4 diagrams.
Personally I think the franchise should have three types of DMU, 175 and 158 for the fast services and 155 or 156 for the slower routes.
Portion working might be a good idea. If accessible toilets weren't so huge it could allow you to use 153s on some Swansea services, so modifying my ideas above you could do:
1tph Swansea - Carmarthen (all stations), 156/155 with a second 156/155 as a portion to/from HOWL every two hours
1tph Swansea - Whitland (all stations except Kidwelly and Ferryside) formed of a 156/155, alternating between Pembroke and Milford with a 153 attached as a Fishguard portion every two hours. I haven't checked this though, the Pembroke train might come back in the wrong hour
If you do this it might be worth keeping a few 150s which would be restricted to the Swansea - Carmarthen stoppers and also run new services from Swansea to the Gwaun-Cae-Gurwen branch in the other hours to the HOWL train, which would still be a 156/155.