Just to add my couple of pennies to this discussion, first, let me say that Mr. Blabey has the "bona-fides" in the shortline biz, as he states. He's been at it (quite succesfully, may I say), since some of the other posters were running Lionels around in circles.
Second, the RRB has a long reach-- a certain railroad in Connecticut that I have some familiarity with, has common-carrier obligations, and is a common-carrier that hauls a very occasional car of freight (average is less than one per year). About 99% of the operations are tourist-excursion-museum. Yet, any paid staff (such as part-time paid flaggers for protecting construction projects over the railroad, etc) are under RRB juristiction, and the railroad pays the small amount into those part-timer's RRB accounts.
If any Class 2 or 3 railroad that is a common-carrier does not pay into RRB, they are in violation of the law, plain and simple.
"You can't always get what you want, but if you try sometimes, you just might find you get what you need."