Trailer-on-flat car. A Trailer-on-flat-car, or piggy-back car allows two 28.5-foot (8.69 m) trailer pups or one semi-trailer up to 57 feet (17.37 m) to be carried. Like well cars, these usually come in articulated sets of five or three. A longer TOFC (trailer on flat car) is usually an 89 ft (27.13 m) car.