Yeah, most game engines like Portal's uses absolute speed relative to the coordinate system (which doesn't change when the coordinate values change), in addition Portal technically doesn't actually implement "wormhole type" portals and instead superimpose a clone of the polygons near both portals behind the other (to preserve expected object collision behavior around the portal) plus doing tricks with virtual cameras, so if you fixed the bugs with moving portals then it would be A.

But if you implemented proper relativistic physics with proper wormhole type portals you'd get B.

The game code works more like A (it bugs out and the object bounces off the portal surface, but it uses a world-fixed coordinate frame that would match A for behavior).

Ah, I see what you are saying. They apply the velocity of the object again after teleporting rather than the difference between the velocities of the portal and the object. Thus the velocity of the train would be ignored. Well, B is wrong simply because the game engine doesn't rotate characters in the teleport because that would ruin character physics. So B is wrong twice.