ADAAG 10.3 Fixed Facilities and Stations. 10.3.1 New Construction.
New stations in rapid rail, light rail, high speed rail, and other fixed guideway systems [...] (13) Where it is necessary to cross tracks to reach boarding platforms, the route surface shall be level and flush with the rail top at the outer edge and between the rails, except for a maximum 2-1/2 inch gap on the inner edge of each rail to permit passage of wheel flanges. [...] Where gap reduction is not practicable, an above-grade or below-grade accessible route shall be provided.

“Public entities must maintain in working order equipment and features of facilities that are required to provide ready access to persons with disabilities. Isolated or temporary interruptions in access due to maintenance and repair of accessible features are not prohibited.”
--DOJ Title II Technical Assistance Manual