the car cannon was essentially a pile of train-hoppers in the same position. You touched it with your car/bike/heli/whatever and it flung you at a ridiculous speed across the end of the runway. It worked in 0.2, but afaik it didn't work properly in 0.3. That and it fucked up the LV Airport race(?).
It doesn't really matter though, because that was 0.2. 0.2 didn't have functions to set/get velocity of vehicles, whereas in 0.3 you can; all you'd have to detect is whether they're in the right position and just boost their velocity a lot. I'm not sure whether it would be allowed back though, it was fun, but more of a distraction and it caused less people to fight. But on the other hand, new players might stick around because the car cannon is some funny shit! :p