A road or railway can impact quite a large area, due to fringe effects. It can cut migration paths. The most valuable nature reserves are the ones that are completely untouched, and a long way from any human activity. On the other hand little islands urban forest are just a waste of real estate (and a great source of wild animals on roads). Wildlife "corridors" may be a useless waste of space, or incredibly useful ... I'm not too sure. I guess it depends on how the animals use them.
Hopefully won't do too much damage, though. As you say, Alaska is really big.
Rail lines aren't really all that wide.
Compute the fraction of Alaska and Siberia's land from which you'd be able to see or hear the trains going past and it's... quite small.