Tracks are easy to fix and an engineering car will carry the materials to fix a break onboard. Its literally shovels to fill in the hole with dirt and a new piece of rail. You would need to physically, with tractors and men, dig up and manually remove the entire track, rather than something you can "blow up". Bombs have a hard time closing airfields, train tracks are even easier to repair.

You can blow up bridges and tunnels but using explosives on tracks are very ineffective.

Generally not every train is armored, you send it when you know there will be hostiles near the tracks, IE precicely because the rails have already been blown up and you are sending an engineering crew to repair the tracks, you would use the armored train to protect your repair crew and spring any traps waiting at the broken section.