I know I'm a moron but isn't this just purely visual? Like wouldn't this be the same as doing it on the ground? Still looks really cool regardless but as far as degree of difficulty isn't this just pretty much as hard as doing it anywhere else? I know this is extraordinarily difficult stuff anyways and it's still impressive, I just don't really get the point of the train other than just kinda looking neat cause of the frame of reference.
Off the top of my head, since he's not moving, his momentum is very different than doing the tricks on the ground. He probably had to get a feel for the difference (notably the difference in air resistance, again, since he's not moving forward).
My guess is the relative distance traveled is greater than usual for some of these things (train moves at a steady speed while he's in the air, while an equivalent ground jump he'd have a velocity that would be decelerating).