The "Emma Sweeny" was portrayed by Rio Grande Southern #20, a narrow gauge locomotive built in 1899 by Schenectady Locomotive Works (not Baldwin, as stated in the film). For the film, the locomotive was repainted and fitted with a false smokestack, headlight, and other features to make it appear older. (This was a common practice in Western films, since true period-correct steam locomotives were often hard to find.) Shortly after filming completed, #20 pulled the Rio Grande Southern's last train ever in November 1951. The locomotive was then stored for many years at the Colorado Railroad Museum. In 2021, after several years of restoration work, #20 was returned to operating condition, and now runs regularly at the museum.
Listen closely at the end, and you'll realize that the names Johnny Behind-the-Deuces (Dan Dailey) and Kit Dodge Jr. (Anne Baxter) have given their daughters are actually the names of the actresses who play the girl dancing troupe in the film.
Marilyn Monroe, who plays one of the chorus girls, was seeing (on the side) one of the extras on the set at the time of production. This extra, named Berry, was also the film's uncredited still photographer. This was verified during the episode of Pickers Like it Hot (2018). Old photos that were discovered during the "pick" with the son of the photographer confirm Marilyn was going out with this photographer named Berry.
For the scenes showing the locomotive off the track, a lightweight full-size wooden replica was built, which, according to a retired fireman, was "so accurate it could fool the experts". This wooden model later changed hands several times and was eventually used in Petticoat Junction (1963) as a studio stand-in for the Hooterville Cannonball. The main Cannonball locomotive was Sierra #3, a much larger standard-gauge Ten-Wheeler, and the wooden model can be easily spotted because its second and third driving wheels are much closer together. After "Petticoat Junction" ended, the model was repainted in its original Emma Sweeny livery and put on display in Jackson, CA, where it remains.
The Tomahawk & Western Railroad is fictional, as are the towns of Epitaph and Tomahawk. The train scenes were filmed on the Silverton Branch of the Denver & Rio Grande Western Railroad near Rockwood, CO. This location was also used in several other Western films, and is now part of the Durango & Silverton tourist railroad.