[burmanm]

If you look at the map you will see that there isn't multiple tracks coming from Russia to Finland. Some of them were even designed to be blown up if necessary (such as the Salla rail tracks).

Finnish rail roads are mostly north-south bound (or west of Helsinki) which are not helpful to Russian advances. The only way for them to transport weaponry would be through east-west bound (near the border) and there isn't many. It's easy to take such out and they would not impact our infrastructure at all as they're not heavily used (if at all since eastern part of Finland is economically the weakest link anyway).

It's quick and easy in the end to destroy. Rebuilding them under artillery fire isn't easy.