Is it that umbrella train thing?
If it is, then basically the way to do it is short horizontal streaming while moving a bit forward and back. Err wait, I can't explain it better. I just do this: go to the direction on the train that's incoming but put yourself sufficiently in the middle so that you minimize the need to move horizontally; only a few bullets actually pass the middle-horizontal path since the trains crowd the sides too much.
If it is, then basically the way to do it is short horizontal streaming while moving a bit forward and back. Err wait, I can't explain it better. I just do this: go to the direction on the train that's incoming but put yourself sufficiently in the middle so that you minimize the need to move horizontally; only a few bullets actually pass the middle-horizontal path since the trains crowd the sides too much.