Bullets don't move at the speed of light.[1] Shoot a bullet backwards off a bullet train, that bullet's ground speed is less than if shot from the ground while stationary. Similarly, shoot a bullet forward off a bullet train, that bullet's ground speed is more than if shot from the ground from a stationary position. Shoot bullets backwards off a jet moving Mach 3 (just sayin') those 1700mph bullets will still be moving in the forward direction of the jet at 600mph relative to the ground.

[1] They should use rear-facing lasers!