Because the frame of reference moves with the train, the velocity measured will be 0 mph.
The frame of reference is where the observer is at. Basically, it would be the zero of our coordinate axis (which may move or not).
In our case, Alice is sitting on the train, so our frame of reference (which is Alice eyes) is also inside the train, moving along with it.
Then, because our frame of reference moves at the same speed and in the same direction as the train, she would see that the train does not move, so from this frame of reference, the velocity of the train is 0mph.
(A similar thing happens when you go inside a car, and in that frame, you see the car being still, and all the things around you moving backward).
If you want to learn more about frames of reference, you can read:
https://brainly.com/question/17548508
