Using the IMU seems hard as true signal of the train's gradual acceleration is likely drowned out by the noise of vibrations and the person's natural movements.

Using the IMU seems hard as true signal of the train's gradual acceleration is likely drowned out by the noise of vibrations and the person's natural movements. I guess every time the train is stopped, you can calibrate the bias of the IMU if the user is standing/sitting sufficiently still, but even then the dead reckoning would drift a lot.