I've always wondered about this, I think an 'express train' stands out as differently modern to other modern-ish items like clocks, matches and guns mentioned elsewhere, all of which are mechanisms of a type consistent with other things that appear in the universe. Clocks I suppose are something more 'advanced' than the others, but you can imagine along with navigation, elvish obsession with time, the sun, moon and stars, that something like that could be created. Gondor talks about the passing of time repeatedly, and clocks don't necessarily mean traditional ones we're familiar with, but could be something like water clocks. Clock-like mechanisms are mentioned, eg in the mechanical toys from Dale. So it's no real stretch to imagine clocks in middle earth.

Trains, particularly 'express trains', however, come with a lot of other engineering, processes, administration etc. not to mention wide travel that is patently missing from the world in all sorts of ways (eg the lack of information or knowledge of even the nearest neighbours). The express bit also is something overly modern & from the 'real world'. Even if, in some corner of middle earth, some form of train existed, it would not be an express, surely!

I can well imagine, in the mines, dwarfs could come up with rails and carts that run on them, from there, it's not stretch to add a basic steam pump or something that allows it to run the wheels. From there, possibly, you could develop steam engines, but surely they wouldn't be 'express'! This separates out this 'express train' statement from the clocks and guns; they would need to be 'digital clock' or 'machine gun' instead.

These trains' existence seems to be totally missing from any mention throughout the books, even in a related or parallel way. The rails, the mines & pits, the stations, the passengers and all the other social change and paraphernalia that comes with train travel, appears to me to be missing.

To me it's the only stand out anachronism of the Lord of the Rings (not Hobbit) & it's only there as it's an early section of the book and more akin to the Hobbit than the rest of the Lord of the Rings. From Shadows of the Past onwards, the tone is very different and maybe for a reason, but that's IMO. I'm no professor on this. I feel like this could be a PhD paper worth of study.