Relativity
Remember that what affects a body in motion is relative changes in motion. We're all travelling thousands of miles an hour already through space, and we're fine
In other words, it is acceleration or deceleration which we actually feel, not the constant speed we're travelling at. The only exception is when you can feel the consequences such as air resistance. So, if this train were to accelerate gradually, lets say at the same rate as a car would (but for longer) - it would expose you to forces no stronger than a car.
Also, Jet lag has nothing to do with how fast you travelled - jet lag refers to being exposed to the time difference when you go from one country to another (so what was your night becomes day - so you're tired).
