C Plate and Other Questions - Trains Magazine

The railroads have various standardized Clearance Plates.

Plate B - you won't see it identified on cars, as I think, that plate defines the limits of cars the can be used in unrestricted service ANYWHERE on the US rail network.

Plate C - is nominally higher (wider/longer) than Plate B and will impact things if operated at a location that only clears Plate B

There are increasingly larger plates up to Plate H, which I think, defines loaded 20'2" Double staks as well as 20'2" autoracks.

The Plates are defined in the Official Equipment Register (as well as it's on line version).  UMLER records also have a byte for the Plate of the car.