Simple place a unit between to enemy forts on the railway line. When the train comes at the start of an enemy turn your unit will attack the train.
This will start an FPS section. Killing enemies doesn’t matter in this section all you need to do is get to the front of the train. Kill anyone you have to buy for the most part you can run past most enemies. I tend to stick to the top of the trains as less enemies and less things you can be snagged on if you take a few to many hits and worry you may die slow down and corner peak back and forth to kill the enemy’s. You only need to get through I think it’s 5 carriages.
I don’t think it makes it easier or harder depending on your army size just a blanket amount.