Train tracks are fixed in place. It wouldn't be difficult for a single fighter jet or bomber to make said rail lines useless.
Won't happen, as that would be used as an argument by Putin. And that's the fucked up part of this conflict. If the seperatists are pushin forward, it's ok, but if Ukraine would launch an offensive to reclaim a city it would be a green-light situation for Putin to roll in with the core of Russia's army. Whatever they do, they loose.