2020 problem can only be solved now by derogations until light rail conversion is completed.
I think it all depends on whether the class 769 plan, which was supposed to see the 150s made PRM compliant, comes off. If we assume that the 150s, 175s and 158s will all be compliant by the deadline, and two 142s are (indirectly) seen off by the 769s, the franchise will be left with the following non-compliant stock:
- 13x class 142
- 15x class 143
- 8x class 153
- 3x mark 3 set
Due off lease by 2020 are 14x class 156 and 16 class 170s (both from Anglia and ScotRail) and possibly an unknown number of 185s (22 sets are expected off-lease but not all prior to the deadline). If, as I hope, the new franchisee has secured a lease on those 156s the 153s can be sent away and (by cascading 150s) the 142 fleet reduced to 10 units. I think the supposedly available 170s total 45 vehicles, which would have to replace 50 Pacer vehicles. The number of units might not quite add up, but I expect the number of seats will given that 170s have longer carriages than Pacers. That just leaves the mark 3s; but that's a small fleet to derrogate (until mark 4s are available to replace them?)
If Wales does take these though, EMT are stuck with their 17 class 153s and 15 class 156s (although the latter may be compliant, are they?), so somebody is going to be in trouble. Maybe we will see ex-London Underground stock with pantographs and batteries after all (and, as long as they fit a toilet and restrict them to the Merthyr/Aberdare-Barry-Bridgend route*, that might even be a releif).
* I pick that one because I think the 6-car routes (Treherbert & Rhymney to Penarth) should be electrified in-full as a priority then get some modern 3-car unit-end-gangwayed EMUs (379s and 458s spring to mind as unit-end-gangwayed and coming off-lease, but the formations would need reshuffling) to run in pairs on those routes. Maybe fit batteries to those two so that (as single 3-car sets) they could run Cardiff-Cheltenham and Maesteg - Ebbw Vale too.
We don't know if widespread light rail conversion is planned; I'm still metaphorically crossing my figures that heavy-rail electrification will go ahead (though sadly due to cost it probably won't be the entire ValleyLines network; but maybe they'll play a long game and have a (slowly) rolling programme). I may be remembering incorrectly, but wasn't there a press release promising that the valleys would remain available for freight?