Rails are usually solid and fixed, but here they "dance in air," as if they've come loose from the ground. The opening image sets a dreamy, slightly unreal tone. The tracks don't say where they lead, and that silence seems to be the point. The creosote, the dark preservative that coats railway ties, is described as keeping "what is unsaid" safe. So the tracks become a kind of storehouse for things people haven't spoken.
The speaker seems to see the tracks as a way of moving words "into the unknown," slipping between trees and eventually going under oceans and past space. The final line, where the tracks "hold birds for all to see," is a quiet surprise. Something built and heavy ends up holding something free and light. The mood is contemplative and wistful, with a sense of wonder. The speaker doesn't appear sad about the mystery. They seem comfortable with not knowing the destination, and with the idea that unspoken things can still travel and be seen.