Definition: The meaning of the term "road number" is: a serial number used by a railroad to uniquely identify its locomotives and cars. Each locomotive or car is given its own road number unique to the railroad that owns it.
Model railroad manufacturers frequently offer their locomotives and cars with multiple road numbers for a particular railroad. The manufacturers research railroad records to be sure that the road numbers they use actually belonged to a prototype of the locomotive or car being numbered (usually).
Some model railroaders are very particular about road numbers, considering it very bad form to put two locomotives or cars on the same layout if they bear the same road number. However, as a practical matter, if you want to run a train with 40 box cars in a particular railroad, it is very difficult to assemble a collection of 40 cars each with unique road numbers. Very particular modelers will repaint or decal road numbers to make them unique.
Model railroad manufacturers frequently offer their locomotives and cars with multiple road numbers for a particular railroad. The manufacturers research railroad records to be sure that the road numbers they use actually belonged to a prototype of the locomotive or car being numbered (usually).
Some model railroaders are very particular about road numbers, considering it very bad form to put two locomotives or cars on the same layout if they bear the same road number. However, as a practical matter, if you want to run a train with 40 box cars in a particular railroad, it is very difficult to assemble a collection of 40 cars each with unique road numbers. Very particular modelers will repaint or decal road numbers to make them unique.