Find a station or maybe an historical Pub or pot luck.

Accessible Train stations in London - Heathrow Terminals 1, 2, 3

All the London railway stations , is on the Piccadilly line

Heathrow is now on the new Accessible Elizabeth Line. It is a bit slower, but a lot cheaper, than the Heathrow Express.
Trains run every fifteen minutes towards London (Abbey Wood). See timetables at TFL for accurate details.
All in all, the new Elizabeth line is amazing. Absolutely brilliant.
The new Elizabeth line trains at Heathrow are level to the platform here, and no boarding ramp is needed.

AccessAble Access Guide :

There is full Lift access between street level and platforms. Escalator access also available.

General description : See wiki

More comment :

This symbol on station page suggests a recommendation - you tell me otherwise, please.