49 CFR Part 37 - Transportation for Individuals With Disabilities at Intercity, Commuter, and High Speed Passenger Railroad Station Platforms; Miscellaneous Amendments

(f) For purposes of this part, level-entry boarding means a boarding platform design in which the horizontal gap between a car at rest and the platform is no more than 10 inches on tangent track and 13 inches on curves and the vertical height of the car floor is no more than 5.5 inches above the boarding platform. Where the horizontal gap is more than 3 inches and/or the vertical gap is more than 5/8 inch, measured when the vehicle is at rest, the horizontal and vertical gaps between the car floor and the boarding platform must be mitigated by a bridge plate, ramp, or other appropriate device consistent with 49 CFR 38.95(c) and 38.125(c).