Buster Keaton wanted to use the real locomotive "General", which was at the Nashville, Chattanooga, and St Louis Union Depot in Chattanooga, TN. The railroad initially permitted him to do so, even providing him with a branch line to film on. However, when it became known that the film was to be a comedy, the railroad withdrew permission, and Keaton had to look elsewhere.
The first try at getting the cannonball to shoot out of the cannon into the cab caused the ball to shoot with too much force. To cause it to shoot into the cab of the engine correctly, Buster Keaton had to count out the grains of gunpowder with tweezers.
For the scenes with the opposing armies marching, Buster Keaton had the extras (which included 500 Oregon National Guard troops) wear the gray uniforms of the Confederacy and march in one direction past the camera, then he had them change uniforms to the Union blues and had them march past the camera in the other direction.
The final battle scene sparked a small forest fire around the river. Buster Keaton, his crew, and the extras stopped filming to fight the fire.