First of all it's not just so easy to destroy infrastructure in a way that can't be rebuilt quickly; thousands of miles of train tracks would be difficult to destroy. This is happening all over Ukraine.

Second, blowing up your own country's rail infrastructure means you can't use it, either, which means you lose an advantage you have that your trains can move on your rails but your enemy's cannot.

Russians aren't stupid, they know that the enemy will try to destroy the tracks when retreating, so they train to fix/bypass the problems quickly.

That includes some transportable improvised bridges ready for deployment.