It's not tunnel expansion but line expansion that is the issue. Right now, Miyabi is the only one who can make those engines run. the Duke wants to invest in technology that he will be able to use even if Miyabi disappears right now.
What I was mainly implying with my comment was that the "magical" power statement doesn't make sense. The inset pictures, describes the magnitude of what he can achieve, but not how others can't do it. The only thing that he has over standard people with skills, is speed and scale(where that magic power comes into play, but isn't a deal breaker). Nothing he has done is impossible for other people to replicate. It's not like Miyabi has ever said, the design of anything was going to be secret. The design of the "Mole Express" isn't highly technical. It was probably made in conjunction with the Apprentice blacksmiths (Again, he is an architecture buff and doesn't have a recipe for any of the pieces). The only weird thing is the Wind Enchantment Engine Block. Probably made with Iron or some other metal/metal alloy. The manga implies enchantment is a rare skill but not impossible to source. Karon(Karen)/Crafters/possibly other magic users, can probably enchant just not with the speed and/or accuracy that Miyabi is capable of. (Ch14 pg18). Again, he can leave an Enchant Diagram for the specific enchantment design. He obviously isn't going to the Duke's House every day to redraw the enchantments on the Hot/Cold Water Blocks. They're just charged by his servants.
And Again this is a prototype. The Engine Block can and probably should be broken up into less technical pieces for easier/Non-Miyabi construction/maintenance. Just based on the crude explanation, you could probably cut the number of "engine blocks" by half, and just make a Wind Direction changer/Engine Block rotator instead and just have 1 block. (Pointing Back, Front, or Neutral[pointed orthogonal to the directions of movement, or even just blocking the hole.]) In terms of charging the blocks, you would want multiple people to charge it, instead of just 1 Miyabi. Or, have multiple Engine block segments that would be less powerful individually and thus be charged separately by multiple people.
If his bath fails due to improper maintenance after a few years, then the Duke will go back to paying someone to heat water for him.
If the train fails due to improper maintenance after a few years, then hundreds of people onboard may die.
All that depends on the final design for the system. But assuming Magic Enchantments don't explosively fail, the primary issue is you would have a runaway train with an engine block that wouldn't be able to slow the train down. Which Miyabi wouldn't be able to "fix"/rescue people anyway, since he's not riding the thing every time it moves. For an abnormal Engine Block, your final train design would need a method to either eject that core, or isolate it so it doesn't provide movement. For the train itself you would need to have crash redundancies at both ends of the track: Impact barrels, water, external friction generation, a literal pond/lake to sink into, whatever. And it probably needs some method to separate the Engine Car from the other Train Cars. So, let's move on from the Train part. The other maintenance issues could be from the Track. Again, you don't need/want Miyabi to inspect/replace those parts either. His Monopolistic Rental Rate for that would also be astronomical. In the real world, we have normal people without magic fix that stuff. It's not that difficult for skilled laborers.
For possible Cave Ins, as I mentioned above, Miyabi offers speed and scale of work, but also probably at an insane price. You would have regular people do inspections of the tunnels themselves, and any repairs. Miyabi's powers makes the initial construction cheap and quick, but the actual work can be done by average people on a normal time scale.
And currently, the planned train is mostly for Cargo, not passengers. So probably at most dozens of casualties. Hundreds, is probably a significant portion of the population of these cities. And they would not be riding these trains all that often. Only Miyabi is thinking that it could be a Commuter Train. Primarily, it would be merchants traveling with their cargo to market.
And I never mentioned it, but it's such a cute add-on, that Mel's Cabin Crew Cap, has a Small Cat Logo on top.