Gangways only really serve a purpose for trolley services, where a buffet is provided in one unit, or for revenue protection staff, and perhaps in cases where trains split or use SDO. To be quite honest passengers are often reluctant enough to move to the next coach, yet alone to the next unit, and once the train is sufficiently packed it becomes difficult if not impossible anyway. The SR fitted gangways to their outer suburban units, right back to the CORs, because it enabled them to have some units with buffet facilities whilst others did not, e.g the BIGs, BEPs, REPs, 4RES etc. BR fitted them to most Sprinters to allow a trolley service, and for paytrain operation or ticket checking with units in multiple. I think 317s were fitted in relation to DOO (they were afterall the first DOO trains), and would probably have needed at least end doors to run to Moorgate anyway. 318s and 455s just carried on the same design, but from then on it reverted to normal. 375, 377, 458 and 450 were built to replace gangwayed CEPs, CIGs and VEPs and run on the same services. Whilst the buffets had long gone, many still ran with trollies. Also quite a few services split and join on route, and there is some SDO.
The irony really is that these days suburban units are often through gangwayed, wheras Intercity and Regional units rarely are, yet the former have far less use for gangways than the latter. If leaves situations where crews need doubling up, or half the train will be without buffet or trolley facilities, or won't see a guard for much of the journey.
