It was impossible for me to hear this story without thinking of one of my favorite, favorite songs,
"Trains" by Porcupine Tree--probably because the song features musical elements of human voices imitating trains, which is where the story ends.
That derailment aside! Great story. I've enjoyed watching this one grow up from a wee little baby drabble to its final form. I was all ready to pull my favorite line, but DerangedMind beat me to it in the second post. I'll add that I loved Dave's comments on this story being a love letter to those who take up the burden of duty for the sake of the people they love. On another level, I think this was a story about oppressive power structures, especially because of its weird Western setting, where the trains may symbolize the people who are REALLY low on the social ladder--perhaps local Native Americans or other PoC, forever locked onto a track that dictates their lives may only go one direction, so that the whole machine keeps moving. There was something beautiful in their empowerment, how one day they just up and leave and refuse to play that game anymore, and disappear into the wilderness to seek their own fortune.
But the tragedy for Bose is that nothing else has changed for those who haven't left. The same power structure is there, and it dictates that a sacrifice is necessary to begin with. This is why, for me, the magical realism of the ending works extremely well. The magic isn't "trains with legs". It's "trains can become like people, and people like trains", the ability to exchange destinies because of a system that demands someone play the role, or else everyone perishes. It's all about, "Who would take my place if I did?" It's not a good, fair, or just system, but given that, Bose achieves something noble in recognizing that someone has to pay the price, and refusing to make anyone else face that same decision.