There is a glitch that causes bumpers to penetrate through enemy bumpers to their entire dimension. E.g., the Hatchet, which is 5 blocks long, should be able to go through a bumper 3 blocks thick (or maybe even 4), such as the Train Plow, and do damage to a part behind it, without destroying the bumper itself. This glitch occurs at certain high speeds, especially when boosting.
I discovered this when using my boosted Clarinet car. I record my battles with that weapon, so I was able to replay and review the precious few instances that happened. In those cases, my car’s cabin wasn’t protected by a wide enough wall of bumpers, so even something like a Train Plow, or the Bastion’s plow on an enemy car, was able to get through when I rammed an enemy vehicle head-on, doing increased damage to my cabin as a result of the high speed, causing my car to explode.

Fig. 1. Never happened with this car. Notice the Gun Mounts between the cabin and the Incisors, as well as the Shock Absorbers above them.

Fig. 2. Started happening after making the subsequent cars more compact by removing the Gun Mounts and mounting the Incisors directly on the cabin.
Maybe it has to do with the rate of registering part movement. So if a bumper travels at a high enough speed, its position, hence the striking end, will be registered behind an enemy’s bumper that would block it below that speed. I don’t remember what speeds it happened at, but each time I was boosting and going very fast. Probably at least 130 km/h. I seem to remember that one time it happened when I rammed a Bastion head-on, at over 200 km/h. But ramming a Train Plow was enough too.
Also, I would sometimes lose the front wheels, despite them not making any contact with the enemy vehicle, IIRC. That was a somewhat common occurrence, with the car in Fig. 1.