I really think that it would be meaningless to try to reproduce the actual rules of underrail into a pen and paper RPG. Really complexe and the rules take advantage of the fact that it's played on a computer.
Even though, if one tries to make it into a pen and paper RPG, it would more likely be close to a tactical board game.
What could be intresting though, would be an oversimplified version of the rules, using mostly the univers, that mix of "magic" and technology. This could be intresting and worth trying to make something.
But that's actually creating a whole new different game. A bit like turning a movie into a book.