It could cripple the economy temporarily if all the carriage drivers, horse breeders, and carriage manufacturers / maintenance crews go out of business. It will take time for people to both adapt to and learn how to handle the new transportation repairs and maintenance.
Plus you cannot just have people with no magic ability opperate it like you can with carriages. It effectively shuts out a decent chunk of the population from being able to easily change from being "carriage drivers" to "train driver".
Add to it that prices for shipping would drop dramatically, and escort services would no longer be necessary either, and there would be a significant immediate economic impact.
But all of that is just immediate impact. Knowing the long term impact could either have it make the economy boom, or end up crippling it too hard before it can have a chance to take off again.
It's a single destination transportation method. The actual transportation/economy fallout is extremely low. 30,000 Karons to run, means the actual cost is, terribly high. And as such is only suitable for transport of high value time sensitive items. The primary fallout is going to be any "Post Towns" that are on the way. They're going to suffer from the drop off of shipments that rely on them to provide logistics and some adventurer escorts. But again, this isn't going to be a mainstream travel method. And, as already stated Angellium doesn't have a ton of specialized resources that need that transport in the first place. All the current plans don't mention any other exits/entrances. Only non-stop Express train service. So, everyone else, is still going to be on horses and carriages to transport things to and from places that aren't specifically Angellium or the Capitol.
And in terms of military potential, it's almost 0. Miyabi, is the entirety of the military problem. Mole-san, is the only person who can make tunnels quick enough to be an invasion issue. Everyone else, is going to take years if not decades, to make a route. And in terms of troop transport, other than an initial push, the underground entrance is really easy to target and stop, once it's discovered to be a problem.
And as a reminder, Miyabi already mentioned his inability to create a Steam Engine, an item that does not exist in game, that he doesn't actually fully understand to make himself. Chapter 31, page 17. He's an architecture buff, not a mechanical engineer. Given enough time, he might be able to remake it from first principles, but that's easily years of research and work, not the limited time scale he was given here.
I don't know about Giorgi's social standing, he could be a lower noble, but he's still subordinate to the Duke. So he has limited maneuvering potential to go against the Duke without external political support. His primary friction point is going to be his ostensible control of the Production Guild. But, that's under the Duke. If he's going to suffer because of decreased control/output, the Duke is the ultimate decision on that anyway. He's obviously going to lose power to Weiss, but he doesn't have a lot of recourse when he doesn't even employ any of the workers that could fit the job/role.
