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Reporting marks on two CP Rail covered hoppers passing Wisconsin Dells, Wisconsin, June 20, 2004. The left one is CP 388686 and the right is SOO 115239.

A reporting mark is an identification assigned by the Association of American Railroads (AAR) to rail carriers and other companies operating in North America.

An AAR reporting mark is a sequence of two to six letters that uniquely identifies the owner of a piece of railroad rolling stock. The ampersand symbol (&) is not considered part of the reporting mark. When a new reporting mark is created, its first letter is selected to be the same as the first letter of the applying company's name; the remaining letters in a reporting mark are generally derived from the applying company's initials. For example, the reporting marks for which Union Pacific Railroad (UP) applied all begin with the letter U. As companies are merged, the companies' reporting marks are normally transferred to the resultant companies along with the original companies' trademarks and logos. The reporting marks that were assigned to Chicago and North Western Railway (CNW), for example, are now assigned to UP which purchased CNW in the 1990s.

Marks that end in X are assigned to rail car owners that are not common carrier railroads themselves (typically these are private car owners, leasing companies or railroad museums). Marks that end in Z are assigned to trailer owners/operators for use on equipment that does not have flanged wheels but that will be used on North American railroads in intermodal service. Marks that end in U are assigned to container owners/operators for use on equipment that will be used in intermodal service. Marks that end in any other letter are assigned to railroads for use on railroad equipment in interchange service.

Where multiple railroads are listed on a reporting mark, their order indicates the order of the mark's assignment; the succession is usually through mergers or buyouts. When a railroad is issued a reporting mark, if it merges with another railroad, the surviving company is permitted to use the prior company's mark, but most railroads typically retire reporting marks of older railroads that they absorb. A reporting mark that is retired is colloquially referred to as a "fallen flag carrier".

Due to this list's size, it has been split into subpages based on the first letter of the reporting mark.

• Standard Carrier Alpha Codes
• Association of American Railroads

• Association of American Railroads
• Establishing and Expiring a Reporting Mark (Railinc)
• Trailer, Chassis and Private Marks (Railinc)
• FRA Guide for Preparing Accident/Incident ReportsPDF (714 KiB) (reporting marks on pages 169 to 185)
• Reporting Marks Search
• Reporting Marks (Comprehensive, but incomplete, summary, based on The Official Railway Equipment Register (also includes histories))