Answer: You are going at 0 m/s, because the air resistance has long since burned the flesh from your bones. But seriously, if anyone with more knowledge of relativity can answer this question, I’d love to hear about it!
Hypothesis: the length of the train appears to be zero from the point of view of an observer on the ground, so you are still traveling at lightspeed relative to the track.
Or, how about this: relativistic time dilation means you don’t perceive time at all if you’re moving at the speed of light, so you actually can’t move relative to the train because you have no time to move in!
Man, I should really learn some actual science about relativity before attempting posts like this.