Maybe the common denominator is just cars. The regulators are fully absent - they care about the shape of your headlights, but do they care that SUVs have them mounted at the height of mirrors for every other car? Do they care that cars have absurdly overpowered engines that have no imaginable use? Do they care that trucks have fronts so high it's impossible to see a kid walking on a crosswalk and when you hit it, it's hit head-height and thrown under the wheels?
None of these things would ever fly on a train or airplane.