All of the energy that is not given off in the form of light, heat, or sound, must be absorbed by Vehicle 7. The energy absorbed by Vehicle 7 is the same amount of energy absorbed by Vehicle 1 and by Vehicle 2. Since the resistance to deformation of all three vehicles is identical, the damage must be identical. While all of the proceeding arguments used the Law of Conservation of Energy as their foundation, almost identical arguments can be made using the Law of Conservation of Momentum. The difference between the approaches is that the Law of Conservation of Momentum does not involve damage.
Energy can result in damage or motion.
Momentum can only result in a change in the velocity vector, if the mass is not altered. Since all of the above collisions are collinear, inline collisions, there is no force that could cause a change of direction except the opposite direction. Therefore, the delta V resulting from the four collisions can result in a change of direction or speed or both. The collisions are collinear, with no restitution. There is no force to separate the vehicles in these collisions, and therefore they must have the same velocity.