If a priest by mistake uses the blessing of a train for a car is there really a benediction places on the automobile or does the car in question still need to be blessed?
Yes, I think you are good to go, by whatever locomotion it is. After all, the Greek ἄμαξα is employed in some Latin neologism choices for vehicles like trains and cars, though for cars I think raeda is more common.
Keep in mind that Rituale Romanum, while an official prayer book with authentic texts for blessings, is still self-described as being a model for other, local or particular rituals. The blessings are meant to be used and even adapted.
Speaking of adaptation, I once blessed the hardware for a knee replacement using the blessing for mountain climbing equipment. It seemed to work pretty well.
Meanwhile, the point of the blessing is the intention of the priest conferring the blessing. If Father made the sign of the Cross blessed your car, you can be pretty sued that he blessed the car.