A reporting mark is an alphabetic code of one to five letters used to identify owners or lessees of rolling stock and other equipment used on the North American railroad network. The marks are stenciled on each piece of equipment, along with a one- to six-digit number, which together uniquely identify every such rail car or locomotive. This allows the cars to be tracked by the railroad they are traveling over, which shares the information with other railroads and customers.
Standard practices 
The Association of American Railroads assigns marks to all carriers, under authority granted by the US Surface Transportation Board, Canadian Transportation Agency, and Mexican Government. Under current practice, the first letter must match the initial letter of the railroad name. As it also acts as a Standard Carrier Alpha Code, the reporting mark cannot conflict with codes in use by other nonrail carriers.
Marks ending in X are assigned to companies or individuals who own railcars, but are not operating railroads; for example, the TTX Company (formerly Trailer Train Company) is named for its original reporting mark of TTX. In another example, the reporting mark for state-funded Amtrak services in California is CDTX (whereas the usual Amtrak mark is AMTK) because the state transportation authority Caltrans owns the used cars. This is why the reporting mark for CSX Transportation, which is an operating railroad, is CSXT instead of CSX. Private (non-common carrier) freight car owners in Mexico were issued, up until around 1990, reporting marks ending in two X's, possibly to signify that their cars followed different regulations (such as bans on friction bearing trucks) than their American counterparts and so their viability for interchange service was impaired. This often resulted in five-letter reporting marks, an option not otherwise allowed by the AAR.
Companies owning trailers used in trailer-on-flatcar service are assigned marks ending in Z, and the National Motor Freight Traffic Association, which maintains the list of Standard Carrier Alpha Codes, assigns marks ending in U to owners of intermodal containers. The standard ISO 6346 covers identifiers for intermodal containers.
When the owner of a reporting mark is taken over by another company, the old mark remains the property of the new company. For example, when the Union Pacific Railroad (mark UP) acquired the Chicago and North Western Railway (mark CNW) in the 1990s, it retained the CNW mark rather than immediately repaint all acquired equipment. Some companies own several marks that are used to identify different classes of cars, such as boxcars or gondolas. If the acquiring company discontinues the name or mark of the acquired company, the discontinued mark is referred to as a "fallen flag" railway. Occasionally, long-disused marks are suddenly revived by the companies which now own them. For example, in recent years, the Union Pacific Railroad has begun to use the mark CMO on newly-built covered hoppers, gondolas and five-bay coal hoppers. CMO originally belonged to a predecessor of the CNW, which passed it on to them, from which the UP inherited it. Similarly, during the breakup of Conrail, the long-retired marks of the Pennsylvania Railroad (PRR) and New York Central Railroad (NYC) were temporarily brought back and applied to much of Conrail's fleet to signify which cars and locomotives were to go to CSX (all cars labeled NYC) and which to Norfolk Southern (all cars labeled PRR). Some of these cars still retain their temporary NYC marks.
List of reporting marks 
Because of its size, this list has been split into subpages based on the first letter of the reporting mark. Railinc Corporation, a subsidiary of the AAR, maintains the active reporting marks for the North American rail industry. Railinc offers a free online look-up of reporting marks and other industry reference files through the Railinc IRFi website.
See also 
- UIC Country Code – A series of prefixes used on rolling stock identification numbers by members of the International Union of Railways (UIC) to identify the country of origin of railway rolling stock
- UIC wagon numbers
- Automatic equipment identification
- Nakina.Net – Reporting Marks – Unofficial incomplete compilation of reporting marks from the 1920s to the present day, as listed in the Official Railway Equipment Register. (Does not include U & Z markings.)
- Railinc Industry Reference File Look-up – search Railinc's free reporting mark look-up by markings (current marks only)
- Association of American Railroads (AAR) Reporting Marks