The force of gravity by classical physics (which are good enough for this discussion) can be calculated as
F = G * (m1 * m2) / r^3, where F is the force we're looking for, G is the gravitational constant, m1 and m2 are the two masses in question, and r is the distance between the two.
So, answering your questions:
1. because the force is inversely proportional to the cube of the distance, gravity actually decreases as you go away from Earth.
2. Well, you can put another force on yourself, double that of gravity. I won't go into that, though, as I doubt that's what you're talking about.
3. the "angle" won't make a difference, the distance between the two bodies will.
4. You'd be travelling at a speed of however fast you were running plus the speed of the train. As I assume the train is on the Earth, then the distance between you and Earth's center of mass won't change, so gravity will remain constant.
Gravity will be ever so slightly stronger on the train because it has more mass than you; however, the difference is neglible. Gravity is a force at work on astronomically large bodies, on smaller bodies it is far too weak.
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