The rules are TooFatLardies Through the Mud & the Blood, which is nominally a WW1 "large skirmish" set of rules but they're flexible enough to take in Russian Civil War, Back of Beyond and other silliness.
The train is basically just a vehicle with one machine gun and light/medium armour. We'll be adding some train-specific movement rules, but nothing too fancy. At the convention in ten days it might also have a protected gondola with an infantry section onboard.