This is just a pet peeve of mine, but every time I read this, I really dislike the "train" part, because I know a little bit about actual trains, like not a lot, but enough to know how stupid it is to make something that big even with magic. Even me with my surface level knowledge can see some extremely obvious issues.
1. The weight of this thing, because of it's sheer size the entire track and wheels would need to be made out of some shit like mithril or whatever, which wouldn't be cheap at all.
2. Power, how would you produce enough power to move it? I know there are spirit crystals, but before, they were barely enough to power a jeep, and considering the context given, this thing has definitely been running before Mira reintroduced high quality crystals into the world. And coal is also completely impossible, because heating enough water to produce high enough pressure is simply not feasibly achievable, and even if you did produce enough pressure, how would you distribute it to the pistons before the steam cooled down too much?
3. The maintenance. How would you maintain the moving parts on this? The pistons alone would be a pain because of their sheer size making it nigh impossible to properly check it without completely taking apart the piston. Wheels would also be difficult, because even if you made them out of some incredibly strong material, they would spend all their time moving against an equally hard material, making the wheels deform, and fixing wheels on actual trains is already a pain, so again the size would make it extremely difficult to even take off the wheel. We have no idea how the power source works so I can't say anything about maintaining that, but there are still a lot of moving parts like all the linkages and bearings, that all need regular maintenance.
4. The coupling, how would you connect something this big to those massive passenger cars without just snapping the couplers. And you can't even make them very tall, because if you did you would need to make the track completely flat, which is impossible without elevating the whole thing.
5. Pressurizing the brakes on the cars. I'll admit this is not a very good point because the brakes could just work on magic, but there is also the thing with having brakes that are even strong enough to stop that thing.
Anyway this thing is a worse idea than Schwerer Gustav was, even considering magic. And calling this rail based form of transportation a train is an insult to actual trains and all the people who made them.
I know that in the end it's just a story, but it's a pet peeve and it has annoyed me every time I read the story and I just wanted to get it out of my system.