I seem to remember a few years back figures exceeding 200,000 riders per year. This was likely when they ran 4 trains a day during those big years of the 80's. I have no idea where they are today.
I remember back in the 80's or 90's I saw figures for the D&S over 200K, and Strasburg over 200K, and White Pass over 400K. I think those are your big 3 tourist haulers in the U.S.
Today, I would say its
(#1) White Pass and Yukon
(#2) Srasburg RR
(#3) D&S
I think there were a couple of years where Ohio Central was close to 200K when they ran their Sugar Creek operation, but there may be another line out there somewhere that can hit the 200K mark, that I am not aware of at the moment, not counting amusement parks!
Greg