Derailing a train by tearing up or blowing up the tracks is an age-old trick, and it definitely worked on armored trains as well. But train operators are supposed to look out for that sort of thing and throw the brakes before hitting a patch of missing track.

Armored trains were MOST useful for protecting regular transport, like supplies or troops ferried back and forth near an at-risk area just behind the front lines - a normal train that gets spotted by an enemy plane, for example, is in for a world of hurt - an armored train might not even be worth the time (and exposure to anti-air guns) to try.