It may be too low, but I'd imagine the app could also look for other Transit users nearby and as long as one of them has the appropriate sensor, this might be sufficient. Probably still not something you want to rely on since it would be pretty annoying to have a feature not work when you're alone on a train.
> the app could also look for other Transit users nearby
That seems unlikely in an environment with no cellular / wifi signal. Theoretically possible, but expensive for battery and probably disallowed by the OS.
