" Their use was discontinued because modern road vehicles became much more powerful and offered more flexibility, and because armoured trains were too vulnerable to track sabotage as well as attacks from the air."
It wasn't quite true in case of World War II. Of course, post-war main battle tanks and precise jet aircraft strikes made armoured trains obsolete in full scale conflict, but in World War II they were still useful and formidable opponents. Note, that Germany had less than 10 half-improvised armoured trains in 1939, much weaker, than the Polish ones, but they evaluated the idea as useful one, and from 1941 until the end of war they operated several dozens of armoured trains, most of them newly built. The Soviet Union operated roughly counting 100 armoured trains during the war, also most of them newly built.
From among German trains, there was not one destroyed by the aircraft (while on the move, at least - one was bombed in Kovel). In case of Soviet ones, there were more destroyed by the aircraft, but still it wasn't great number.
While it is true, that probably most trains were lost being cut off from main units and surrounded (due to destroyed bridges or time needed to repair a track), a sabotage was not a great threat. The train wasn't moving fast, so in case of derailment or impact mines, only a front flatcar was derailed or damaged. Remote mines were used by partisans rather, than regular troops - but they would have to know, that the armoured train would ride that way, and when (it's much easier to blow up a regular train). All in all, it would probably be an occupied area, and the damaged train could be recovered.