Why are freight railroads separated into classes?

U.S. freight railroads are usually identified as Class I, Class II or Class III. That can be helpful for several reasons.

The Surface Transportation Board (STB), an independent federal agency that oversees the economic regulation of freight railroads and deals with issues related to railroad rates and rail service, has divided the freight railroads into three categories based on their operating revenue. According to the American Short Line and Regional Railroad Association (ASLRRA), those...