old boxcars. Common reporting marks are FEC, CP, SOO and KTTX. The ATTX cars, which feature non-sparking grips and sides, are built for hauling dangerous goods (ammunition, flammable fluids, etc.). Spine car A spine car is a car with only center and side sills and lateral arms to support intermodal containers. Trailer-on-flat car A Trailer-on-flat-car, or piggy-back car allows two trailer pups or one semi-trailer up to 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 car. In the past, these carried three trailers which are, as of 2007, almost obsolete, or one large, , two or trailers. As intermodal traffic grows, these dedicated flats are in decline. Most have been modified to also carry containers as well. One notable type is Canadian Pacific Railway's XTRX service—dedicated five-unit flats that only carry trailers. Skeleton car Similar to the spine car except that it is designed to carry lumber or utility poles, a skeleton car is composed of a center sill and lateral arms only. No deck, sometimes no side sills and sometimes no end sills. Idler flatcars In some marine services, the linkspan between a ferry or barge and its dock is very weak. In order to avoid loss of cargo or heavy locomotives, an old flatcar (which is usually the lightest car available) is used as a bridge between the locomotive on the dock and the cars on the ferry or barge. Idler flatcars are also used in oversize freight service, as loads such as pipe often overhang the ends of most standard-sized flatcars. Empty flatcars will be placed on both ends of the loaded car. This protects the cargo ends from damage and ensures that the loads don't bind and damage the ends of adjacent cars. Often a flat car is placed directly in front of a crane ("big hook") in order to: provide a way to remove a wrecked car from a crash site. provide a way to store new or removed rail from a work site. allow room for the crane's boom while in transit to and from a work site. Idler flatcars are also used to mount one kind of coupler on one end and another kind on the other end. This is called a match wagon or a barrier vehicle. See also Bogie bolster wagon Conflat Containerization