I'd be surprised if any two coach EMUs are built in future (given the need for a DDA toilet etc) - just my personal hunch (rather than anything official)
Good point that. Half the 2-car units would probablly have to be toilet-less (and only used for extending other units) if you go down that route.
AFAICR there are around thirty Pacers with ATW, so if you assume that the 33 DMU off-peak requirement for the Queen Street services means half a dozen additional Pacers needed (due to doubling up/ maintenance/ peak extras) then you should have enough to replace all ATW Pacers and FGW Pacers by doing the Queen Street lines (no need for Maesteg/ Ebbw Vale etc)
It's a while since I made that guestimate, but I think I had more than 8 150s released, but with ATW retaining the others for improving services in the north and south-west of Wales. I think it also created enough slack for ATW's 8 153s to be reduced to 4, so I guess you could be right that you could get rid of all Pacers outside Northern without wiring Ebbw Vale and Maesteg.
I don't think a 2-car 315 would be practical as it would require a complex and expensive conversion of either a DMSO into a DPTSO or a PTSO into a DPTSO. A build of 3-car and 4-car units seems to make the most sense (or just a pure build of 3-car units with SDO for the platforms where doubled up units are too long).
I wasn't talking in terms of 315s with that suggestion, just generally in terms of the lengths of unit that would seem to sit well given the platform lengths. In particular I'm still thinking in terms of purpose-built units similar to 377s, but if cascaded EMUs are what makes the case work then do what you can with them. If 315s are what the valleys will get, then I think it is probablly best to stick to 4-car formations (no corridor connections makes multiple working messy).
3/4 coach, corridor connected, so able to run as 3/4/6/7/8 coach, best of both worlds?
Somthing like 3-car 377s (the corridor connection is a must, for strengthening while allowing passengers to move between units while moving) would probably do the job nicely. Some 4-car units might be needed for the routes with 4-car platforms though.
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That makes sense. Ideally I'd like your stand along Cardiff - Cheltenham service to be tagged on to the XC Nottingham - Birmingham "short" (giving two Nottingham - Cardiff services an hour) but that would need more 170s (which aren't exactly spare).
Concentrating 158s on a shorter Cardiff - Cheltenham service makes a lot of sense, allowing ATW to keep the 90mph units on suitable services (given the fact that they don't always have enough faster DMUs for the Manchester/ Holyhead trips) - good thinking
Ideally Cardiff - Cheltenham would be electrified using the just-mentioned 377s and running through to Swansea as the Swanline, but some LM-style 172s just doing Cardiff - Cheltenham (possibly extending to Birmingham, but that's moving out of the outer-suburban band that Cardiff - Cheltenham proabablly is, making 170/172 units less suitable) would do the trick too. That would release those three 158s for new regional express services (I'm tied between Cambrian mainline hourly and Cardiff - Pembrokeshire via Swansea district line).