Difference between Class 1, Regional, Short Line, etc?

What is the difference between a Class 1 RR, or Regional, or a Shortline ? Does track mileage constitute a Class1/Regional/Shortline, or is it traffic tonnage?


Thanks,

Mark

Mileage has nothing to do with class, the revenue generated does.

see:
http://en.wikipedia.org/wiki/Class_I_railroad