Though I understand that a light armoured train would be useful for protecting railways from raiding forces I am curious as to how the heavier armoured trains were used. Wouldn't it be extremely easy to stop one by simply breaking up the tracks?
As a side question I get the impression that the designs for armoured trains were never standardised instead you have a large variety of different models. Could anyone shed a light on this?
Armored trains served several functions. The first, as you mention, is anti-partisan warfare. They protected supply line against raiding parties and guerrillas.
The second is to be "Johnny on the spot", being sent to hotspots which are in severe need of additional firepower to repel an offensive.
The last, which is the rarest, is use as an offensive weapon. As you say, pulling up the tracks is all you need so there are few times it can be done. There are a small handful of incidents in World War I, but the main time you see this is during the Russian Civil War.
That is the very brief of it. For the very long of it, I would direct you here, as I have written extensively on this before. It covers the development from the American Civil War to post-World War II, and I go in-depth on the various types of trains of the principle users. But even this is far from totally comprehensive, so if you have any follow up questions, please do let me know and I'll do my best with 'em.