Please Explain.......

This is straying somewhat from the original subject of this thread, but it goes to a question that has bugged me for quite awhile......
What, exactly was or is the requirement for common carrier status? Specifically, how did small interurbans, street railways and "dummy lines" avoid common-carrier status and ICC regulation? They carried passengers, and general freight. Today, some short lines operate under what is called "contract carrier" status which relieves them of some types of regulation. Did early light-duty local lines have some similar catagory?
Of course a common carrier could participate in interline rates and eminent domain takings, but it seems like many of the smaller narrow gauge lines would have been better off without the onerous regulation of the ICC. A number of private carriers, esp. the logging roads also offered limited passenger and general freight service to the communities they operated in, was this legal or done "under the table"?
Thanks, Steele or whomever can shed light on this question.
J