wow. what a deep question 
one definitely requires a PHD in physics to answer it.
i remember when i was about 10 years old i was asked this question: a candle is under a glass dome in a moving car. the car suddenly stops which way does the flame swing ? the answer is that the flame goes in direction opposite to that in which the car was moving. and damn, i feel stupid for not having had figured this out at the time this question is not hard but it really tests reasoning ability
Even cooler, vasyachkin: Put a helium balloon in the passenger seat of a car. It floats up and touches the ceiling. Then make a narrow left turn. Observe the balloon.
Ain’t science great?
Gravity and acceleration are the same; the balloon is buoyant in the opposite direction to the force - forward acceleration in a car presses you back into your seat in the same way that gravity presses you down onto the ground, a balloon is buoyant in the opposite direction for the same reasons.
Yep, Mangetout. And that’s cool.
Ain’t science great?
Did you know that if you took all the veins out of a man’s body and laid them end to end, that man would die?
Ah, an Empiricist.
Given that the OPs long answered and we’ve moved on to aphorisms:
Did you know that if you laid all the economists end to end they wouldn’t reach a conclusion?
But they’d be happy.
If, pound for pound, a giraffe could jump as high as a grasshopper, he’d avoid a lot of trouble.
“If all the girls who attended the Yale prom were laid end to end, I wouldn’t be a bit surprised.”
Dorothy Parker
Is that a fact?
Back to the OP: The simplest answer is that the train isn’t moving. At least, in the reference frame of the guy in the train, it isn’t. And if the train is going at a constant speed, that reference frame is just as good as the reference frame of the guy standing outside on the ground.
Always use the easy frame of reference.
Is spitting your chewing gum over the keyboard a new one…?!