History: Rail Transportation In The United States

Rolling stock reporting marks
Every piece of railroad rolling stock operating in North American interchange service is required to carry a standardized set of reporting marks. The marks are made up of a two- to four-letter code identifying the owner of the equipment accompanied by an identification number and statistics on the equipment's capacity and tare (unloaded) weight. Marks whose codes end in X (such as TTGX) are used on equipment owned by entities that are not common carrier railroads themselves. Marks whose codes end in U are used on containers that are carried in intermodal transport, and marks whose codes end in Z are used on trailers that are carried in intermodal transport.
Typically, railroads operating in the United States reserve one- to four-digit identification numbers for powered equipment such as diesel locomotives and six-digit identification numbers for unpowered equipment. There is no hard and fast rule for how equipment is numbered; each railroad maintains its own numbering policy for its equipment.