Ok, from my experience:
The car going outside the track has to weigh less than 500kg.
The car ramming the other car has to weigh a lot (works best with tank car, but have seen X1's work because of their sheer speed.)
Park your lightweight car at the end of a long straight, sideways against a wall.
Let heavy car ram you as hard/fast as they can (works best if you aim for front or rear of car, to concentrate the force at one end) and it also makes it even easier if someone in an X1 pushes the tank car from behind.
You should see your car go halfway through the wall, just drive the rest of the way and you're off to freedom!