Personally, I strongly suspect that the NPS did
not place the train back on the bridge to appease train geeks' desire for a cool photo. Rather, I would suspect it was done so as to place the artifacts in-situ in their historical context. The fact that the exhibit exists in an area that is exposed to numerous tourists who
aren't die-hard railfans and who wouldn't necessarily seek out a display if it was somewhere else also strikes me as a bit of an intangible benefit.
Now, I'm not saying that environmental deterioration isn't a thing...nor am I suggesting that the artifacts wouldn't last longer under roof or in building. However, to call the entire endeavor a stupid waste of money seems very shortsighted - especially in the greater context of preservation, and how to engage the public with history and artifacts.
"All very good things, and all very bad things, don't last very long". Keeping the artifacts in the most immaculate condition for the longest amount of time may, actually, be secondary to exposing as many people to the artifacts in-context as possible. But what do I know? I'm a dummy on the internet and my opinion don't mean much!