I need to say this, just because I kind of know what the thoughts are
So, I'm assuming the movie is set in the old west, either the 1880's or maybe a few decades beyond that.
there is only so much you can do to backdate a real locomotive. So many things get changed over the years, that without a complete rebuild, its hopeless.
Look at 346 for example. What would it take to back date it to its original form? Its got extra compressors, different plumbing, electricity, backup light, different stack, I mean, so much is different
Movie's don't have the money to bring a locomotive back from the dead just for a production, unless your Ted Turner. I though Disney did good on Lone Ranger, at least the replica's they built looked halfway decent.
Give them some credit, at least their trying. Not everyone is a railfan. Their using a real, breathing engine in a movie, rather then CGI or a replica...thats something to be proud of.