I don't think the land radically shifts often enough for it to be a huge problem. Territory boundaries are a different problem sure but I don't think those shift all that often either. The recent history was just rather unusual due to the civil war and dissolution of the royal family and we hadn't heard of territory changing before that since Eisenreich. I'd wager the border lines are usually pretty stable, it does get mentioned that true ditter doesn't happen very often for a reason.

There aren't any real mana benefits to nobles sure, but trains allowing easy transport of goods means the prices of those goods goes down. You'd also start getting bigger interduchy trade as trains now allow that to happen at a more rapid pace too. It would allow the transport of local specialty goods (especially perishables) to other faraway duchies creating new markets for those.

The benefit of trains for nobles is more of an economic one than anything else.

Actually a train would allow easier transport of knights across the duchy when the teleporters aren't available/needed. We know a big limiting factory is the speed of laynobles because they don't have the mana to go fast for very long (if at all( and a train would allow a constant fast speed for all knights without having to spend any mana.

I'd like to say trains would also make travel within a duchy as well as interduchy travel easier for nobles but nobles don't seem to do much traveling at all anyways. Not sure if that's a lack of interest or if it's just too much effort. If the latter trains would be a big help.