I'm not aware of any apps that do this unfortunately, although it would be very useful. Sometimes the individual transit providers website offers this functionality though (such as Transport for London). Apple Maps also includes the location of steps, elevators and the like, but you have to zoom in to each station to see. It also doesn't include information on whether the elevators are working, or connections (older cities like London sometimes have stations that are accessible only if you're connecting, or on certain lines etc).

Unfortunately it seems there's quite a lot of work to do in this area. IIRC Google have added accessibility information to quite a lot of Maps recently, maybe they'll expand it to Maps as well.