|The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (August 2013)|
In North America the mark, which consist of an alphabetic code of one to five letters, is stenciled on each piece of equipment, along with a one- to six-digit number. This information is used to uniquely identify every such rail car or locomotive, thus allowing it to be tracked by the railroad they are traveling over, which shares the information with other railroads and customers. 
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 with the letter "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 agency (Caltrans) owns the equipment used in these services. This may also apply to commuter rail, for example Metrolink in Southern California uses the reporting mark SCAX because the equipment is owned by the Southern California Regional Rail Authority—which owns the Metrolink system—even though it is operated by Amtrak. 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 with the letter "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 North American reporting marks
Because of its size, this list has been split into subpages based on the first letter of the reporting mark. Railinc, 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's Freight Rail 411 website.
In the United Kingdom, prior to nationalisation, wagons owned by the major railways were marked with codes of two to four letters, these codes normally being the initials of the railway concerned; for example, wagons of the Great Western Railway were marked "G W"; those of the London, Midland and Scottish Railway were marked "L M S", etc. The codes were agreed between the railways and registered with the Railway Clearing House.
- Automatic equipment identification
- 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