After he finished this film, Martin Scorsese screened it for John Cassavetes. Cassavetes, after seeing it, hugged Scorsese and said, "Marty, you've just spent a whole year of your life making a piece of shit. It's a good picture, but you're better than the people who make this kind of movie. Don't get hooked into the exploitation market, just try and do something different." Scorsese's next film was Mean Streets (1973).
There was a rumor that Roger Corman's wife Julie Corman obtained the film rights to the story from Bertha Thompson herself after tracking her down in a San Francisco hotel room; she never actually met Thompson--because Thompson wouldn't open the door-but that rumor wasn't true. It may haver been a pre-release publicity stunt or maybe even a trick played on the Cormans by the true owner of the story, author Ben L. Reitman; the afterword in the fourth re-issue of the book "Boxcar Bertha" explained that the book is a work of fiction, and that the character Bertha Thompson was an amalgam of at least three women that Reitman knew, but was mostly modeled on a woman named Retta Toble.
The Reader Railroad was the actual name of the railroad where the train scenes were filmed. It opened in 1889 and is still in business, used at various times for freight, tourism and movie service. At the time of filming it was still regularly using vintage equipment, most notably steam locomotives.
The train sequences were shot first and they took about a week. This was done to get the most complicated element of the production, working with a moving train, out of the way first.
Martin Scorsese: as a john who is just finishing dressing himself when he asks Bertha if he can spend the night.