Question

If you are sitting in a train, looking out the window, and you see another train pass, what is your frame of reference to tell which train is moving

Answer 1

Look at the ground maybe.. or the sky. Other objects you can see that that don't move but rather you move past. So anything that stays in place. So not people, not the other train. Look at the ground, the tracks, the sky.

Answer 2

The train you are sitting in would not be moving because the other train passed you. Or your train could just be moving slow. Hope this helps! :D