Relative motion and train travel

Trains are a classic example of relative motion. If you can see the train next to you and you notice motion, is it you or is it them? In theory you should be able to tell if it is you that has started moving because you feel the acceleration  as you are gently pushed back into your seat if you are facing forwards. But train acceleration is often very low so it is common to mistake the movement of the train next door as your train moving. The laws of Physics say that if a car is coming towards you at constant speed and you are driving towards it at constant speed, you add the speeds together and it would be as if you were stationary and it was coming towards you at that bigger speed.