This isn't a button cheat but instead a way to beat a tough level.
In the train mission, it's easy enough to get to the end but it's pretty frustrating finishing it.
When you finish defending the base and your men are parachuting down to the tracks, hotswap to the front man.
After manning the machine gun, open fire on the train for as long as possible.
Drop the gun before the train passes you so the next man on the line can have more fire time. Repeat this 3 times.
By now you should be the last guy on the line, forget the gun and use the under-the-gun grenade launcher.
Try to hit the front of it but as long as you hit the front car it will cause damage. If you do enough damage with the machine guns, it should only take 2 or 3 grenade hits.