Not exactly free, but many charter's will rotate rider's through the cab, depending on the RR's rules, of course. This seems to be more likely on museum and tourist lines, although I think the D&S and C&TS don't currently allow it.
The Nevada Northern rotates cab riders on their winter steam spectaculars. The 40 is one sweet riding loco and with the high drivers, the engineer and fireman sit way up high above you when you are standing on the deck.
Back when the Ohio Central was still running railfan weekends, they would sell raffle tickets for $1 to get a cab ride between runbys. People usually would buy 6 tickets for $5.
If you do get a cab ride, be sure and wear long pants and a long sleeve shirt and be wearing sturdy shoes. Especially in summer, steam leaks are invisible and you can get burned if you aren't protected. Gloves are a good idea as well. Be sure and follow the crew's directions to stay out of their way and to stay safe.
Michael Allen