What is a Railcar Reporting Mark? Here is a good explanation, From Wikipedia:
“A reporting mark is a code used to identify owners or lessees of rolling stock and other equipment used on certain rail transport networks. The code typically reflects the name or identifying number of the owner, lessee, or operator of the equipment.
In North America the mark, which consists of an alphabetic code of two to four 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 it is traveling over, which shares the information with other railroads and customers.  In multinational registries, a code indicating the home country may also be included.”
Hence, for privately owned rail equipment, the “Mark” is the four-letter code stenciled in several specific locations on each railcar, that identifies the car owner, or in some cases, the lessee of the car if the owner allows. Private company (not railroads) cars have Marks that end in “X”. There is a master registry of Marks, maintained by the Association of American Railroads, and the process of establishing a new Mark for your company is not difficult.
To run your own (or leased railcars if the lessor allows), you need to place your Mark on each car. All private cars that are registered in the system are associated with a Mark and a unique car number. The combination (Mark and Number) designates a specific car and all of the specifications for the car are associated with that record. Owners can then track each car, noting where it is and whether it is loaded or empty. Also, the railroads will perform key maintenance associated with the wheels, bearings, brakes and related “rail-related” components of the railcar and invoice the Mark owner based upon approved rates.