don't put sewage tanks next to the exhaust? was this something the designers of the voyagers missed?
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for double deckers, they might not be that far away, with wagons getting ever lower, why not move underfloor equipment elsewhere and lower the floor of the carriage with the area above the bogies being standard height with doors and couplers. and of course with the super-low bogies and wheels, there could be a design on the table soon
Double-decking would probably be more practical on the Southern, overhead wire clearance can be decidedly non-standard. Trains would have to be narrower below platform level than continental ones, with the lower deck in a well between the bogies. Doors, wheelchair/luggage compartments and disabled toilets would have to be over the bogies. However, current loading gauges are getting close to practical DD height and width. It's do-able.
Hopefully, the next stage will be a standard local unit for rural services, something like a modified 172 with doors at the ends. There really ought to be a PEP-replacement as well, probably a 378 or something similar, or perhaps a development of Crossrail/Thameslink stock. 313s, 314s, 315s, 455s, 456s, 507s and 508s will be history, as will Pacers and Sprinters on rural services - 172s can replace them on suburban services. A bit later could be a standard outer-suburban unit using newer technology than the current crop, to replace the MkIII-based crop.
Then of course, there's whatever replaces 225s and the remaining HSTs. After IEP, there should be sufficient through services from HS2 and the possible HS3 that there will no longer be any need for extensive on-board services. I'm expecting HS2 to out-compete the ECML on long-distance passenger flows (unfortunately for me, but why should people who live in the Home Counties mind changing to get to Scotland :roll: ). The first post-HS2 generation of inter-city stock will be much more Voyager/Javelin like, since it will cater for shorter-distance flows such as the Leeds and York semi-fasts. So trolley catering, limited first class and shorter trains sometimes running in pairs to reduce costs. It might be called IEP-2.
On freight, which we haven't really considered, I'm half-expecting an a.c. electro-diesel so that it can run trains off the wires at reduced speed. It's hard to remove a container when under the wires, and this will save a lot of shunting moves. Hopefully, there will be some smaller locos as well, or an increased use of MPVs to avoid having Type 5 traction on light duties. Finally, there really
must be a new standard shunter, Gronks looked dated 20 years ago.