When you jump while on the tram ride during the intro, the tram continues to move forward and you do not so it seems as though you are jumping forward/backward. Gordon should just jump “in place” because he is moving forward at the same rate as the tram.
I have a feeling that this is a problem with Valve’s engine since it most likely handles the physics but I figured I’d bring it up.
EDIT: No getpos or status messages.
TO REPLICATE: Replay the opening tram ride. While the tram is moving forward, jump. The tram will continue to move forward while Gordon is in the air.