MikeB-here are some that come readily to my mind.
Walt Disney and Ward Kimball. IIRC, Mr. Kimball was an animator for Disney Studios and had a huge collection of vintage toy trains as well as a 3-foot gauge steamer (the 2-6-0 Emma Nevada from the Nevada Central) running on his property.
For more on Ward Kimball, check out this link:
I've seen photos of Mr. Disney at the Chicago Railroad Fair, held in the late 1940's, inspecting the equipment on display. I believe that it was Mr. Disney's interest in railroads that was instrumental in the train rides being an integral part of Disneyland in southern California.
For more on Walt Disney's interest in railroads, follow this link:
While not necessarily railfans, Frank Sinatra, Joe DiMaggio and Roy Campanella (former Brooklyn Dodgers catcher, until his career was cut short by a tragic auto accident) were well known Lionel collectors. In fact, Joe D. narrated a television show titled "Tales of the Red Caboose", basically a Lionel advertising program, on New York television in the early 1950's.
O. Winston Link, professional photographer by trade, made himself and the Norfolk and Western famous with his photos of the end of steam on that railroad. (See the thread already devoted to him.)
Sportswriter Stan Fischler has also written a couple of volumes on the New York City subways.
Rogers E. M. Whitaker, noted writer for the New York Times, and later the New Yorker magazine, wrote many articles on rail travel under the nom de plume E. M. Frimbo. He is quoted as having said "I became a sports writer...so that someone would have to pay me to go by train to universities in the eastern part of the United States and report on their winter sports". He is credited with having travelled over 2 million miles by train. His book, "All Aboard with E. M. Frimbo-World's Greatest Railroad Buff" is a classic and worth searching out.
Michael Palin, of Monty Python's Flying Circus fame, called himself a "trainspotter" and narrated a couple of programs on PBS involving travel. My favorite, from 20+ years ago was of his travels in England in the "Great Railway Journeys of the World" series.
The late singer Johnny Cash also had an interest in railroads-he had a variety show on television in the late 1960's that always included a segment with railroads. I can still remember the song that started out "Come along and ride this train..."
The late "Boxcar Willie" was another country singer with roots in railroading and I would be remiss if I didn't mention the "Singing Brakeman", Jimmie Rodgers who spent a stint with the Southern Railroad before eventually finding fame in the music industry.
A couple of others who came to mind after my first post...
Ellis D. Atwood of Edaville fame.
F. Nelson Blount who started the "Steamtown" collection.
Fred Rogers of "Mr. Rogers Neighborhood" fame.
Joshua Lionel Cowen-while most wouldn't know who Joshua Cowen is, mention "Lionel" and you usually immediately think of model trains.