I have a long history of making YouTube videos where I strongly argue for opposing points-of-view. For example, despite frequently complaining about the overuse of trams, I still made a video called “The Case for LRT” where I talk about why you might want to do an LRT. Understand the arguments for and against something is something I generally suggest — it deepens your knowledge and helps you make a more compelling case. It also helps you understand that usually there is no right answer, just compromises to be made.
I recently made some videos on the Grand Paris Express’s Line 15, which happens to be 100% underground — in fact, a large proportion of the whole Grand Paris Express (>200km metro) is underground, and even the yard I visited for Line 15 was fully indoors, with a covered link into the tunnels below.
To be clear, fully underground metro lines are nothing new — Paris’ Line 14 is also entirely in tunnel, but what’s so interesting about Paris Line 15 is that it is urban, and unlike Line 14 which is about 14-kilometres in length, Line 15 will have a 75-kilometre tunnelled route. Even high speed railways, such as the Chuo Shinkansen (yes the maglev) travelling under the mountains of central Japan, and HS2 (in notably much less mountainous, but perhaps much more NIMBY-filled Britain), are more than half in tunnels. And who can forget the tunnels linking Hokkaido to Honshu, the English Channel, and southern and northern Europe through the Alps!
When I see the drive towards tunneling happening in much of the world, I do wonder to an extent about Toronto’s Ontario Line — to be clear, a great project, and I do wonder what it might look like had it been fully-tunnelled. In the case of the Ontario Line, this would almost certainly be a worse option, particularly cross the Don Valley where a tunnel would have forced super deep stations on either end, but I do think it’s interesting to ask… what are the devil’s advocate points for tunneling?
It’s expensive, right?
If you’ve followed me for any extended period of time, you’ll know that I frequently talk about why building things above ground is a good idea, and how elevated rail isn’t a blight on cities. One of the most significant reasons for not digging is that building above ground is basically universally less expensive (given we aren’t talking about Manhattan), and costs are a real crisis in the English-speaking world.
But there is a great case to be made that cost need not be a concern with tunneling in places either where construction costs are sufficiently low, or there is a lot of wealth. In such places, the cost premium, or even just the absolute cost, is not high enough to make going underground a bad idea. For example, Seoul is likely to complete its whole high-speed underground regional rail system for less than one moderately long metro line in Sydney or Toronto, or the cost of half a metro line in New York.
Even putting things aside, it’s worth asking what the benefits of going underground are — so we can determine if it’s something we should be pushing towards even in places where it’s not strictly necessary as is probably the case for significant portions of the Grand Paris Express.
The Benefits of Going Below Ground
Probably the most obvious benefit of going below ground is reduced environmental impacts (that is to the human environment). Modern underground rail done correctly is going to have essentially no noise or vibration impacts, and the visual impacts are really just station head houses (which can be embedded in large buildings) as well as some emergency exit and ventilation structures.
More interestingly, going underground allows for more direct alignments with wider curves, and if deep enough, also less grade changes. There’s a real distinction between most of the world’s old subways built along road networks and other right of ways with cut-and-cover construction and most modern metros that do not obey patterns found on the surface and also tend to have far smoother alignments.
The real benefit of the emergence of deep tunnelling — which was first used a scale in London and which has been made almost trivial by tunnel boring machines — is that rapid transit can now be fairly easily build in, around, and under dense urban areas. Some cities like those in Australia have started using giant “sheds” to cover access shafts and portals, which makes it almost impossible to tell underground construction is happening. Every year, new underground projects push the limits and solve new problems in building this type of infrastructure, and the amount of innovation happening in the space is quite impressive.
There are pretty obvious climate and resiliency benefits to tunnels too. While a recent trend is to critique tunnelled rapid transit (and really any infrastructure that is disproportionately about reducing humanities impact on the planet these days!) for its embodied carbon, or the carbon required to create it. The truth, is there is no other piece of transportation infrastructure that can move masses of people with anywhere near the efficiency of mass rapid transit (shallow tunnelling significantly reduces the carbon cost of subways as well, due to dramatically reduced excavation volumes and concrete use).
Perhaps a more interesting conversation to have about the climate implications of underground transportation is its resilience to climate impacts. For example, extreme temperature swings that will wreak havoc on rails and other metal infrastructure (thermal expansion and contraction) can be better moderated underground. The potential for events such as washouts, which can be an issue with heavy precipitation on conventional ballasted, railways are a nonissue with underground systems, and flooding can also be robustly protected against.
It’s not a major feature in my mind, but the potential for underground transit infrastructure to be used for security particularly in times of conflict is obviously real — many of Singapore’s MRT stations are designed to act as wartime shelters, and the London Underground’s deep stations were famously used as shelters during the blitz. More recently stations in the Kyiv metro have been used as shelters from bombardments.
The truth is being underground simply puts a little, or sometimes a lot between the outside world and your infrastructure. That “insulation” has the potential to create a more stable and amenable environment for critical infrastructure.
You might imagine that this “environment isolation” has few practical benefits, but that’s not really true. Already in more extreme cities, metro systems provide climate refuges — such as from the cold in Montreal and from the heat in Singapore. Montreal’s entirely underground metro has shown itself to have another benefit though: trains last much longer when they are not exposed to the elements, and they can be lighter when they don’t need to be thoroughly weatherproofed.
Of course, there are also risks to creating an underground bubble. Floods and fires that do happen underground are much more dangerous and hard to manage because of the restricted access, and the same is true for equipment and structural failures.
Challenges and Solutions
Tunnel are not easy, and while they present a lot of opportunities, there are significant challenges that tunnels bring with them; some of these challenges have clear solutions, while others are in flux.
One big challenge with tunnels is as always — ventilation. While it is true that a big reason tunnelled transport projects in places like New York are so expensive is because of an onerous interpretation of fire codes, ventilating tunnels is an issue — not only when considering smoke during a fire, but also to maintain temperatures. London shows that it can be extremely challenging to solve tunnel heat issues retroactively particularly in dense city centres.
Maintenance is also a worthwhile consideration, particularly because as you can again see from London among other places — rail tunnels can stay in service for a very long time. An above ground or elevated rail right-of-way can in theory be completely reconstructed rather easily (comparatively speaking), but maintaining tunnels in the long term is likely to be a much more significant challenge — especially in unfavourable geological conditions such as the type of place that would require an earth pressure balance machine.
There’s also the issue that people casually treat all tunnels as the same, when they really are not. When “the Boring Company” was a hot news topic, the amount of un-nuanced tunneling discussion I saw was crazy. There are a number of different tunneling techniques which are used at scale today including TBMs, NATM, drill-and-blast, cut-and-cover, the Milan method, pipejacking, and immersed tube. Even within the TBM category there are a number of different types of TBMs used in different situations, with different bore sizes (and different potential uses of those tunnels!) and suitable soils. While the idea of being able to get so good at tunneling that entire metro systems can be build underground easily without arousing any NIMBYism above ground (kind of like an imagined Singapore) is an attractive one, actually getting to that point will require a mastery of a vast array of underground construction techniques.
Another issue is station access. Once you start tunneling much below the surface, going deeper to avoid underground structures and hit better soils is attractive, but super deep stations are hard to access and we don’t seem to have great solutions for this yet. As it turns out, building tunnels has become fairly easy — it’s getting those tunnels to the surface and building structures along them where the huge challenges often lie. “Elevator-only station” still arouses concern, but people also don’t love having to go down super long escalators, much less several switchback escalators. Perhaps this will be less of an issue if future metro networks are entirely constructed deep below ground, and then access is really only an issue at the beginning and end of journeys — but creating better horizontal and vertical flow through underground rail stations is an important challenge.
You also have to consider where to store and maintain trains and how to get the trains there. An underground yard is possible, but the significant volume means a significant cost. Meanwhile, surface yards take up a lot of land and can be challenging to site particularly in dense cities. Hong Kong’s approach is instructive here, with concrete decks built over surface yards that can support high-rise development.
Something else that is a real challenge with underground transit is modification and extension. Adding a new branch, a station, or extending an underground line is a lot more challenging than when a route is above ground. That being said, as transit systems transition to a more modular approach that is less monolithic and which sees passengers transfer between distinct services more often this may become less of a challenge.
Now, the biggest reason I think tunnelling’s challenges are interesting to discuss is that while there is a lot of variability from location to location when it comes to tunneling, it still appears to be less than what we see on the surface. At the same time, the reduction in external factors and considerations you get from going below most everything should ideally make building underground infrastructure a fairly simple process someday. Much like with nuclear power, I believe a lot of the benefits of tunneling are yet to be fully taken advantage of and that the more we do it, the more dividends society will see from being more experienced in this particular endeavour than would otherwise be the case.
That being said, I should say there is one place where above ground basically always wins — the view.
You're forgetting another important impact of above ground vs below ground metros.
The property/core real estate that you loose. Plus a lot of metros still have a noise impact as they run 24/7. I'm in Copenhagen by Ørestad and there's both the airport nearby and the metro. However the building basically fully isolates for the planes but the metros can still be heard.
Where the metro line is build there could be 10.000s of apartment units and other Comercial buildings, as as you mentioned considerably less maintenance. The above ground areas are also the ones most prone to failures and have often cancelations where you need to take a bus to the below ground part of the network.
How come it seems like most of the elevated train problems regarding alignments also exist with cut-and cover, such as being stuck to the street grid?