Not so much the cars as the platforms:

[www.access-board.gov])

(9) In stations covered by this section, rail-to-platform height in new stations shall be coordinated with the floor height of new vehicles so that the vertical difference, measured when the vehicle is at rest, is within plus or minus 5/8 inch under normal passenger load conditions. For rapid rail, light rail, commuter rail, high speed rail, and intercity rail systems in new stations, the horizontal gap, measured when the new vehicle is at rest, shall be no greater than 3 inches. For slow moving automated guideway "people mover" transit systems, the horizontal gap in new stations shall be no greater than 1 inch.
EXCEPTION 1: Existing vehicles operating in new stations may have a vertical difference with respect to the new platform within plus or minus 1-1/2 inches.
EXCEPTION 2: In light rail, commuter rail and intercity rail systems where it is not operationally or structurally feasible to meet the horizontal gap or vertical difference requirements, mini-high platforms, car-borne or platform-mounted lifts, ramps or bridge plates, or similar manually deployed devices, meeting the applicable requirements of 36 CFR part 1192, or 49 CFR part 38 shall suffice.



Amtrak has been allowed to use ramps and such since freight railroads are the primary carrier. However, I think the current push is for all new construction (including LRT and commuter rail) to have full platform access, and the use of gauntlet track or retractible platforms is expected to used as an alternative if possible. From what I can tell, there seems to be a mass of confusion between the various agencies of what the applicable rule actually is. The FRA issued their own interpertation, which is different from the DOJ interpertation of the ADA requirements. I have heard rumors within the last few years of not being able to use brand new platforms beause some agency determined they did not comply.