The rock they dug through for Koralm is, no hyperbole, about as bad as it gets. It's the gnarliest part of what's under the Alps and required them switching back and forth between boring and blasting.

Being two separate tunnels, it also needs twice as much excavation work. It's also ~25x deeper than Toei Oedo (4000ft vs 157ft). At 4000ft the rock itself is 45-50C!

The Koralm tunnel has a different temperature gradient, as the depth is a consequence of a mountain on top of the tunnel, rather than an increased proximity to the earth crust/core.

    > "The undisturbed rock temperature varies from 10 °C, in tunnel sections close to the portals, to 32 °C in the tunnel centre"

32°C is still a significant engineering concern, but not as consequential as 45–50°C.

https://www.sciencedirect.com/science/article/pii/S088677982...

Looks like they reported a peak of 39C in the summer. Either way I figured that would still be pretty miserable, especially if it gets up around 100% humidity.

Assumed they would at least have their own air in the bits that didn't have aircon/ventilation while it was being built. They don't even need to do that anymore! The ventilation systems they used are as advanced and bespoke as the boring machines.

Because they were blasting too, they couldn't utilize full-face pressurization of the entire tunnel to maintain negative pressure to suck all of the fumes, dust, silicates, etc out like they would if it was only boring. That's 1-3kPa, "leaks are jets of air, can pull an airlock door closed hard enough to break bones" territory.

Instead, they have a bunch of dedicated supply and exhaust vents going to the surface (some up to 2m in diameter) and sets of connections between the two tunnels with huge axial fans. It allows them to selectively apply "slight" negative pressure to any of the individual segments when they need to clear them. 50Pa is ~10x what you encounter in a negative pressure highrise. It is described as a "constant slight breeze"

I found this short video on some of the safety features of the finished tunnel. It almost looks "too serious", like something out of a James Bond movie https://www.youtube.com/watch?v=I8trt96huf0

A tunnel on the Kurobe gorge railway (originally used for dam contruction, now partially open to tourists) has reached 160 °C (!!) during construction, but has cooled down to manageable 40 °C since.

https://en.wikipedia.org/wiki/Kurobe_Seny%C5%8D_Railway

Good point about "boring vs blasting". I didn't think about that. I remember reading about the longest tunnel in Japan between Honshu and Hokkaido (Seikan Tunnel). I recall that it was entirely hand drilled due to unusual soil conditions. I wonder if that would still be true today with state of the art tunnel boring machines.

   > Being two separate tunnels, it also needs twice as much excavation work.

Yet another great point. At some of the Toei Oedo stations, you can see a miniature model of the weird overlapping twin tunnel boring machines. So, in theory it is two tunnels, but in practice, it was dug as a single, weird overlapping twin tunnel.

It's also really hard to make the tunnel remain a tunnel over its expected 150 year lifespan - given that it basically runs through a fault line. They had to study and test local geology for about 15 years, build certain sections to expect some movement over time, as well as kit everything out with a lot of sensors.

Overall an amazing achievement, and unsurprising it took this long to figure out!

After seeing some of the safety features in a short video I linked in another comment, I get the impression that this is either going to last much longer than 150 years or something so catastrophic will happen that nothing that could have been built would've persisted.

Geography I guess[1].

Kanto is flat, it's the only region in Japan that could sustain feeding such a massive population and could allow building the first mega city on the planet.

Combine that with the massive engineering and rail experience Japanese have, and it's no surprise imho that combined with favorable geography they could build it quickly.

[1] https://upload.wikimedia.org/wikipedia/commons/2/28/Topograp...

This is interesting analysis. Many good points. Regarding this comment: "first mega city on the planet": As I understand, in the modern era, Beijing was the first city in the world to have one million people.

Really? I always thought it was London in 1801. Do you have a link to the data?

Mega cities are 10M+, NYC was the first metropolitan area to cross this milestone and Tokyo the first city proper.

I'm guessing geology play a big part - Japan is mostly "new" rock, Alps mostly "old".

Sorry, I don't understand your point. Why is Japan considered "new" and European Alps considered "old"?

The Japanese islands are situated in one of the most geologically active regions on Earth, primarily characterized by multiple subduction zones where four major tectonic plates, producing 'new' Earth, emerge.

The Alps are very, very old in comparison.

It is very strange that countries like Austria, Japan or Switzerland have some of the best rail systems in the world even though their bridge and tunnel requirements are huge. In the US building rail on any terrain seems to be more expensive than basically anything one can build in Austria.

Not strange at all! If you want to go by car you must build even more tunnels. Mountainous regions favor rail just like urban areas do. Furthermore, 19th century investments into rail still pay off in mountainous regions, because once you build a railway bridge or tunnel, you are kind of dumb not to use it. In the USA competition from trucks or cars is much tougher.

Note the US has the number one freight rail system in the world by most measures (there are lots of ways to measure this depending on how you want to abuse the statistics). Some of this is Europe has better river routes to work with (I don't know Asia), but some of this is Europe has focused more on passengers to the point where freight is unable to get through forcing trucks.

And still, to drive on highways in the US is to drive between trucks.

Here is a huge/difficult island with only 55K that has tons of tunnels: https://maps.app.goo.gl/abcnXtRNs9QdVkhC8

Population size, density, terrain, etc. have nothing to do with it.

You also cannot directly compare a metro line to intercity rail. That line in Tokyo was like what, 20th they built all in the same terrain, they are really good at this by now. Meanwhile, rail tunnels are usually bespoke projects.