I'll ask again. Does anyone happen to be 'in the know' about whether the issues with shortening the 319s (compressors in particular, as the toilets will need sorting out somehow anyway come 2020) would apply just as much to the other similar EMU classes such as 317s or 321s?....
I don't know them particularly well, but on the other Mk3 based units (class 325 excepted) most of the underfloor equipment is on the PMS (Pantograph Motor Standard), with some on the adjacent DTS (Driving Trailer Standard). This is possibly to make it a standard design to facilitate the likes of the 318 and 320 units. The 319 has no 3 car variant and, afaik, was never intended to have one.
It should be noted that the 319 (and the 325) have more equipment due to their dual voltage capacity.
I cannot say what internal equipment the centre trailer on the other Mk3 units have, but I suspect it is not much for the reason noted above. I think I am right in saying that, until the recent refurbishments for Scottish stock, none of them have a TET (Toilet Effluence Tank), but the 319 does (317/1s were built with them but they were removed when they left the MML).
....I was actually surprised to see the guard ask if anyone wanted SWT as we approached (I was in the rear of the rear set) and escorted them to the front car of the set (the front set was a /1, the rear being /2). He then released all doors on both sets, in spite of the fact that the rear carriage was off the platform. He'd obviously taken the precaution to ensure anyone alighting was in the front 3 cars, but I'm surprised the rule book allows such a procedure.
Grandfather rights I believe, stock can call at stations that are too short if they had done so before current rules came into place. A pair of 150s can call, but, when it gets electrified, a 319 cannot without having SDO fitted (or the platform getting lengthened), despite them being the same length.