Вот ответ на этот вопрос на амовском воено-железнодорожном формуме (англ.)
When the M-1 tank was developed, it was discovered after-the-fact that the Army had no way to effectively transport them by rail. The existing fleet of 120-ton capacity flatcars could only carry one M-1 tank since two such vehicles at a bare-bones weight (i.e., less fuel and ammunition) of approximately 70 tons each would overload the flatcar's capabilities. A new 140-ton capacity was designed and built to carry two M-1 tanks. I haven't kept up with the business, but with the subsequent modifications to the M-1 series Main Battle Tanks, and the added weight that goes with such changes, the DODX flatcars themselves may have also been modified and strengthened to carry the heavier loads.
Other tracked vehicles in the military's inventory, apart from the M-88 series tank retrievers, weigh considerably less and can utilized commercial equipment (i.e., TTX or railroad-owned flatcars) or other DODX rolling stock. In Vietnam, for example, I shipped disabled M-113 armored personnel carriers on US-built 40-foot narrow-gauge flatcars on the Vietnamese National Railway System. They were loaded onto the flatcars using 5-ton wreckers.
The Military Traffic Management Command's Transportation Engineering Agency link will provide a few pictures and some insight into what goes into moving such equipment.
