For instance, there is a standard - colloquially known as the "Western Standard" - which allows for almost-floor level loading of the bottom level of a Superliner/California/Surfliner car. This standard is 15" above the top of rail. As well, the FRA allows for alternative heights to any of the standards in specific cases should certain criteria be met - and it is under this that Utah's Frontrunner has built ~22" high platforms that allow for level loading with the floor of their BiLevel coaches.
As for adjustable gap fillers on the equipment, most of the Bombardier BiLevel cars built for the US market in the past 10 or 15 years have been fitted with powered ramps/gap fillers to meet their accessibility requirements. (Most US properties running BiLevels use more-or-less the same accessible ramp design as was pioneered in Toronto.) I've always found this amusing, as while Metrolinx with all sorts of additional-cost "extras" - larger door windows, powered end doors, additional exterior lighting, etc. - the powered ramps are the one thing that they've not ticked the box on.