You need something to open it, look around. Tip : you can make the last push with only 7-8 zombies in your horde :)
I agree, there is small parts of the road that are destroyed and block the car from going the wrong way but we don't see it very well, i couldnt place a big obstacle because you need to be able to place your horde on the back to push it. This is why i added NPCs between the cars, if you take them, you will move the car by accident and understand there is a "puzzle" . Before that most people missed it.