A silent movie from 1927. IMDB has this (as I type) at number 131 on their list of the top 250 movies of all time. And, yes, it's pretty amazing.
The hero is "Johnnie Gray", played by Buster Keaton. He's a railroad engineer, justly proud of his trusty engine "The General", based out of Marietta, Georgia. He's also sweet on Marietta's prettiest local girl, Annabelle Lee. When the Civil War breaks out, Annabelle insists Johnnie enlist, as her father and brother do. But he's turned away at the recruitment office, being more valuable as an engineer. Through some miscommunication (which happens pretty easily in a silent movie) Annabelle interprets this as cowardice, and breaks off the romance.
Note: A heartbroken Buster Keaton looks about the same as a deliriously happy Buster Keaton.
A year later, the Union has a plan: infiltrate Georgia, steal the General out from under Johnnie's nose, and use it to disrupt the Confederate supply lines and communications. The plan starts off well, except that innocent passenger Annabelle Lee gets kidnapped as well. Johnnie is off to the rescue.
No fancy-schmancy things like stunt doubles or special effects here. When they want a bridge to collapse and send a locomotive into the river below, their "special effects" magic is: collapse a real bridge, send a real locomotive crashing into a real river, and hope you remembered to load film in the camera. (This page says the train remained there for years, finally salvaged for scrap iron during World War II.)
One thing: the "good guys" here are all on the side of the Confederacy. That's a little cognitively dissonant in these days where a Confederate flag is an automatic sign of unmitigated evil. You'll have to turn off that reflex to enjoy the movie, I think.
It might make a good double feature with The Great Locomotive Chase, based on the same true Civil War incident, told from the Union side with Fess Parker as the Union mastermind plotter.