When you are in the rail car in the introduction to the game, if you jump up the carriage moves forward underneath you, jumping forward in the direction the carriage is moving gains little distance and jumping in the opposite direction results in a huge leap. In reality, this would not happen.
This might be a limitation of the Source engine. I don’t think that there’s anything the devs can do about that.
There’s plenty the devs can do about it. It’s just movement code, which is entirely in the SDK.
But does that actually matter?
No, it doesn’t.
Are you telling me Source doesn’t have conservation of motion, but does use the Doppler effect with sound?
Theoretically, they could have the entire map move around the tram 
it was the same in real half life 1 so i think they just did it for the nostalgic feeling.