I haven't made any of the narrow gauge runs, but I've been on some of the standard gauge runs with Motorcar Operators West (MOW), one of the groups that participates in Railfest.
We operate in a group, and are led by an employee, usually in one of their own highrailers or speeders. They are with us to ensure that we are clear of any train movements. They don't turn us loose to operate at our own discretion -- everything is operated in accordance with their rules. And, the railroad collects a fee.
Bill