I hope someone can help me figure this one out, I only remember it from when I was a kid so the details my be a bit off…
Men are running about in what I think was a hidden military base dug into the side of a mountain, they were trying to escape a bomb detonation or something until they find out the bomb didn't o off and that they were safe…for the moment. Then all the men scramble back in the base to a train with a steam locomotive. They all started it up, got it moving and climbed on.
The next scene, on the outside of the mountain, I remember something like a hidden doorway opening up on the side of the mountain and the train came out through it while wood planks flipped up and to the sides right in front of the train revealing the hidden track leading out of the mountain. The planks end with a track bumper breaking in the middle and flipping open to the sides while the train rumbles through.
The train carried lots of war materials, including one very big cannon. At some point the train gets chased by some bad guys who start climbing onto the train and moved up the cars toward the engine. The crews on the train uncoupled some of the cars (including the big cannon) so that they could make a get away, and put explosives on the car with the big cannon.
But when they drove on the explosives failed to destroy the cannon.
One of the crews said something like “we can't let them have that gun” so they reversed the train direction recoupled the train and kept driving on.
Next scene you see a big explosion on the tracks far ahead of the train, realizing that the bad guys had trapped the train and crew, they decided to stop and fight them.
That's all, hope someone can help me out.