The base game is over 10 years old - the program gets updated each year but it will never have Multiplayer.
It did have multiplayer for a while in the form of the "relay" scenarios - basically you drove part of a scenario then when reaching a set objective, could send it over to a person on your steam friends list who could continue the run where you left off. AFAIK the scenarios themselves are still in the game (they can be seen if you go into the scenario editor), but the relay function has been disabled.
