It seems like the lack of level boarding at the vast majority of mainline railway stations, even busy ones, is something that is very difficult to fix in the UK.
It's not really that difficult. but some recent strategic mistakes have been made. We have a more standardised platform height than most European networks, already at a fairly high 3ft. Not all platforms exactly match that clearly but a large majority already do, and it's more achievable in future rebuilds than 1100mm or more. While over the last few decades European operators have been quietly rebuilding platforms and buying new lower-floor trains to match their new standards, the UK industry has seemed to stand back and say we're alright we've got high platforms already. Except that we weren't alright at all and the facts that some small segments of national infrastructure like parts of the Elizabeth Line have been built with extra high platforms, and either Harrington humps or being met with ramps are the solutions elsewhere, are testament to that.
A low floor option wasn't available for the newly developed Bombardier Aventra bodyshell design, first delivered only a decade ago, which has once again locked many parts of the SE into high floor for the life of these fleets. Meanwhile, the Level-Boarding revolution has continued relentlessly in Europe to the point it is becoming quite rare to encounter a non-LB experience in many places. I hope after a few small add-on orders to recent fleets from Derby they will be the last Aventras and Alstom will abandon the design and re-equip the plant to build a new 3ft floor train for the UK, just as they are building for Irish Rail currently with their X'trapolis design in Poland. That new bodyshell will be approved in the EU, so cross-acceptance should be straightforward and that could put Alstom in a good place for many of the upcoming UK regional renewals. The Irish trains will include battery-equipped examples, so in a few years, it could be a well-known, proven design suitable for many UK applications, and hopefully also being built at Derby.
As a new bodyshell design from the last decade or so, Bombardier's Aventra could have been engineered to have a lower floor from the start, but all stakeholders appear to have been institutionally blind to that possibility, despite what was going on all over Europe and further afield. I admit to being gobsmacked by what appeared to be niche solutions a couple of decades ago for low floor on rural branches. I kind of felt there would always be big high floor Eurofimas and ICEs, accessed by multiple steps at smaller stations. Today most trains in service and almost all being ordered new are level boarding at a small family of platform heights standardised under TSIs.
A similar story is now unfolding over HS2 rolling stock. With only three brand new stations, with the higher platforms, the majority of stations served by the new trains will now be existing ones having a traditional British vertical gap with steps at the doors. Alstom or Talgo could have proposed a 913mm solution with their all-trailer passenger accommodation configurations sandwiched between end power cars, but that would have conflicted with another HS2 requirement for distributed power, the Japanese preference for traction architecture, and the high speed power bogies cannot be accommodated currently under a continuous (another HS" requirement) 3ft floor, whereas trailer wheels usually can somehow with wheelboxes in the saloons or at extremities with a shared articulated bogie configuration. Alstom's TGV-M could theoretically have some traction motors and associated systems above the articulated bogies at the car ends; the continuous level floor gangway is actually on the upper deck of these bi-level trains. SNCF have only ordered the end power car configuration so far, and Alstom now claims this is lighter in weight overall, less complex, more reliable and cheaper than traction equipment spread throughout the train. The other major fleet of Avelias being delivered, to Amtrack in the USA, also have end power cars and have unpowered tilting single-level passenger cars between, with a ~4ft floor height matching US high platforms for LB on the electrified parts of the network it will use.
Talgo's Avril as supplied for Spain and also with end power cars, is both level boarding at matching 760mm HS platforms and has a continuous level floor throughout the passenger accommodation, with the inter-car gangway passing between the tops of the two individual wheels of the patent articulated steering wheelsets. These trains could likely be adapted for 913mm platforms in a UK profile bodyshell.
Doesn’t help with the other gap though, just the height.
Modern level boarding trains also usually have moving gap filler steps that can close a horizontal gap. Many can adjust to varying gaps so can also assist on curves where the platfrom has to be set back a little to clear end or centre throw.
