http://en.wikipedia.org/wiki/Flatcar#Intermodal_freight_use
COFC (container on flat car) cars are typically 89 feet (27.13 m) long and carry four 20-foot (6.10 m) intermodal containers or two 40-foot (12.19 m)/45-foot (13.72 m) shipping containers (the two 45-foot / 13.72 m containers are carryable due to the fact that the car is actually 92 ft (28.04 m) long, using the strike plates). With the rise of intermodal freight transport-specific cars, and given the age of most of these flats, numbers will decline over the next several years. Indeed, when the first well cars appeared, allowing double stacking, many container flats were re-built as autoracks. The few “new build” container flats are identifiable by their lack of decking, welded steel frame, and standard 89-foot length. One variant is the 50 feet (15.24 m) car (which usually carries one large container as a load); these are actually re-built old boxcars. Common reporting marks are FEC, CP, SOO and KTTX. The ATTX cars, which feature no spark grips and sides, are built for hauling dangerous goods (ammunition, flammable fluids, etc.).
Kansas City Southern Railway #8985, a flatcar seen in this May 29, 2004, photo, is fitted with fifth wheel coupling hitches for hauling trailers.
A TOFC (trailer on flat car, a.k.a. piggy-back) car once again, is usually an 89 ft car. In the past, these carried three 30 ft (9.14 m) trailers which are, as of 2007, almost obsolete, or one large, 53 ft (16.15 m), two 40-foot (12.19 m) or 45-foot (13.72 m) trailers. As intermodal traffic grows, these dedicated flats are in decline. Most have been modified to also carry containers. One notable type is Canadian Pacific Railway’s XTRX service — dedicated five-unit flats that only carry trailers.