An extremely large, poor-handling automobile with sloppy power steering and an automatic transmission, driven by a person who doesn't really know how to drive.
Road locomotives were common in the 1960s.
Maxine drove to Vegas in her road locomotive.
A piece of Detroit iron. A huge car "driven" by a person who really didn't know how to drive, and who didn't know what a good car was.
Mike's old man always drove Detroit iron. He sure got mad when we drove circles around him in our sports cars.