Second, what are the rules for the artillery on the rail in Savage Station? The scenario on the site doesn't say that the artillery must stay on the rail, but that seems to by the intent. I suspect the original scenarios had some description on them.
Can't help you with the first part of your question but my read (for what it's worth) on the Savage Station rail artillery is that it must stay on the rail. The special rules seem to imply as much how they describe straight ahead movement and allowing 'passage' even when the hexes are occupied (Arty is on the train!)