Consider what happened to all of those domesticated horses that Spirit "freed:" they likely have no knowledge of how to survive in the wild, and the ones released from the locomotive are still in chains as they run into a freaking forest fire..
They were actually running away from the forest fire, which started all the way back where they came from. (The train slid down the mountain). They're domesticated horses; they'll end up back at the place they left, their owners will feed them and patch them up, and when a replacement locomotive comes they'll pull it over the mountain and then have a nice rest before being shipped back East to a comfortable stable. Humans are only Bastards if you're a wild animal who wants to stay that way.
Many of the horses were stolen from the Lakota. They were also released almost a minute before the fire started spreading. They'll survive just fine, although they'll probably head back to the Lakota camp to be ridden again. As for the railroad, the entire camp was destroyed, as well as two locomotives. They lost all their horses too. That's not something rectified for two bucks and a packet of gum. The humans mentioned a six day deadline that is definitely not going to be met.
Also, consider where Mustangs and Brumbies came from: domesticated horses that got loose/were released. Some of them will probably die, all of them will have a hard first months, but they'll probably be able to figure something out eventually, especially if they can find an existing herd they can join. Provided they don't just run to where they consider "home" and that there actually is something there as suggested above.