I think this is the safest way to destroy the Armored Train in HARD Difficulty.
Besides the two weapons shown in this video, Field Gun can also be used to destroy the train. I didn't use them because they were a bit slow to operate (giving the train more chance to hit me) and were also destroyed by the train after some time.
![](https://i.ytimg.com/vi/h1ZWgRwquCs/mqdefault.jpg)