 Subways and metros are usually the same. You have two tunnels, or maybe one bigger tunnel, under the city, with stations in between. And so I often find that a lot of people aren't all that interested in one system versus another, because once you've seen one metro, you've kind of seen them all. Because they do kind of all fundamentally work the same. But back in the early 2000s, one city thought, let's try something different. Very different. And the idea really was so interesting and frankly ingenious that it got a ton of global attention. But like a lot of similar scenarios, this project really has not gone to plan. And the problems it has had rarely get any oxygen because it's such an interesting project. And this is really why my channel exists. To reasonably present something interesting and talk about some of the positives and negatives. So other cities don't just try to emulate the city, but try to understand it. This will be an episode that seriously nerds out on engineering and in-depth subway system design, so just warning you. So without further ado, let's talk about what I would personally say is probably the most interesting metro line or lines in the world. Barcelona Line 9 and 10. So Barcelona Line 9 and 10. Are they two lines? I'd love to hear your thoughts about this in the comments. I'm really of the school of thought that if you have one line with some branches off of it, still just one line. It's not two separate lines. You don't get the credit for building two lines if you only have one central tunnel. But let me know what you think down below. Anyways, if you've seen any RM transit videos, you'll know that I go on and on and on about how we build so many of our subway and railway tunnels far too deep underground. And that the most cost-effective and convenient way to build metro systems is using the tried and true and very old method of cut and cover construction. In fact, I actually think this is one of the major benefits of say the New York City subway over the London Underground, where much more of the subway is built using cut and cover and it's a lot easier to get down to the stations. However, if you followed any transit construction project, you're probably familiar with all of the issues that can come along with building something close to the surface. Utility relocations, roadworks, vibration, damaging buildings and setting nimbies afire. It sort of seems like you should either go really shallow and get those cost benefits or go really deep and avoid all of the nimbies. This is something that planners and engineers wanted to take advantage of for Barcelona Line 9. It would go deep, really, really deep. So deep that it could go right under existing buildings, foundations and all, not to mention any other metro tunnels or utilities. This thing was going to be way under all of that. This is all great, but when you build a really deep metro station, you come up against some serious challenges. You can, and cities often still do, build stations cut and cover. The issue with this is that as your station gets deeper and deeper, you have to excavate and shore up a bigger and bigger pit. And even getting together enough land that you can have the above ground size of a station worth of land on the surface can be really difficult, especially in a dense historic area such as where Barcelona Line 9 runs. Because of this, a lot of cities build stations using mining or other methods, which works exactly as you would expect. The station is methodically dug out from some small access shafts. The issue with this method is it doesn't work all that well in certain types of soil. If you're in Manhattan or Stockholm or parts of Sydney, it can actually be a really good method because the strong rock is really good and self-supporting for the most part. So you're kind of just digging and building your station, easy. But in cities like Toronto and large parts of Barcelona where the soil is a lot more like dirt, kind of sandy, it's a lot more difficult. And in any case, it's somewhat risky, complicated and can be very expensive. So planners and engineers in Barcelona came up with a very novel idea. And this is part of why I have such great respect for the transit engineering capabilities in this part of the world. Obviously for so many metro and railway projects, not to mention even sewers and roadway tunnels, we now use tunnel boring machines. Those giant cylindrical mole-like machines that dig out circular tunnels that you can then put subways in. So what if we just used a tunnel boring machine to create the stations on a metro line? Of course, once you start tunneling, you can exactly easily change the size of your tunnel boring machine. So the idea here was to start with a tunnel boring machine big enough that you could actually build your stations within the tunnel. And then the tracks between the stations will have tons of extra room around them, but that's fine. This is all great, but in and of itself, it's not a solution because there's another big problem. How do you actually get people to those stations, especially if they're deep underground? Part of the issue is that the trend in metro systems is to use escalators to get people down to platform level, which is very comfortable and convenient. And even if they aren't technically always faster than elevators, escalators give you the sense that you're always moving since you rarely have to wait to get on one. The issue with escalators is that when you're building an access shaft down to a deep station, escalators force you to have a fairly wide access shaft because escalators can only be so steep, and if people have to change escalators a bunch of times going down, they stop feeling so convenient. So instead, for Barcelona Line 9, for many stations, access is entirely provided by a series of high capacity, fast elevators, which are placed in a big circular shaft. You still do need to provide emergency stairs, but since they won't be the primary means of access and they might not ever really be used as long as you don't have any serious problems, they don't need to be super comfortable or fast to use. The whole access via a vertical shaft thing works really well because you don't need a ton of land on the surface to build it. Now, if you're curious how the station works within the tunnel bore, which is just the name for the space that gets bored out by the TBM, the platforms are built in a stacked configuration, and this is to take advantage of as much space in that big tunnel bore as possible to give you wide platforms and space through the tracks. The San Jose or Silicon Valley Bart extension project is claiming to take inspiration from Barcelona Line 9 because it's also planning to build stations within a tunnel bore. But super strangely, Bart is planning on using island platforms and on top of the fact that Bart trains are already pretty wide, that means that a lot of the space won't even be used within the station locations. Whereas in Barcelona's case, there's a lot of extra space between stations, but within the stations the space is basically all used up. What's worse is that because of the Bart plan to use a single island platform, even though the tunnel itself is gigantic, because the trains are wide and you're talking about a single island platform, you have what is relatively narrow platforms serving both directions of passengers. It's just not optimal. Now, actually getting between the station and the vertical access shaft isn't all that complicated. A short connecting tunnel is used in this case, and then you can have large staircases and elevators for people going between the two levels of the station itself. This is another advantage of the stack design versus the Bart island platform design. Because of the island platform having tracks on either side, everyone needs to go across and then go down to access the island. Whereas with the stack design, 50% of people going in one direction are already going to be on level with that track when they get to the landing of the main shaft, and so less elevators and less risk of elevators breaking has an impact on riders. Now, because of the stack design, much of the line, including the entire center of the line, puts one direction's trains on one level and the other direction's trains on the other. To be clear, stack stations aren't new, nor are stacked tunnels for trains traveling in either direction. What's new here is building all of the station facilities within the tunnel bore, as opposed to some other way. Now, part of the problem with the stacked approach traditionally is you can't keep your tunnels stacked forever. If there's a problem on your line, you need to be able to get trains turned around, which means getting them between the tracks going in one direction and the tracks going in another, which is difficult when they're on top of one another. You can't simply build a cross-connecting tunnel, as you would be able to if you had two parallel tunnels. Because of this problem, there are all kinds of crazy solutions which have been used to solve this problem. My favorite is the coupled TBM, which has at least been used in Japan, where two circular TBMs are basically linked together, and then they can change their orientation as they're tunneling, so that they can be vertically oriented to keep the profile of the subway line narrow and then horizontally oriented so that you can have cross connections between the tunnels for trains to turn around. The really nice, under-appreciated thing about the big tube is that it's so big that you can actually fit two tracks on both levels, which is kind of how you can have a track and a platform on both levels. And what this means is you actually have enough space to actually construct ramps between the two levels, all within the tunnel bore. Which means that despite being stacked, Line 9 actually has a ton of different locations where trains can turn around over the line, making it more resilient to operational issues. All of this seems to be part of a bit of a fundamental paradigm shift you're seeing in different areas of construction, where traditionally we built structures around their functions, and that really influenced the envelope of the overall structure. We now seem to have a tendency to build a big envelope and then customize what we build within, as you see with Barcelona Line 9, but also as you see with new airports, where you now just see a big terminal building with things like baggage drop-offs and security checkpoints dropped wherever within, sort of like little buildings built within the larger building. Now the truth is that in the Barcelona case, not even mentioning the Bart case, this feels kind of wasteful. You're digging such large tunnels for what are relatively small trains. But the thing is, the TBMs themselves are actually very efficient, at least compared to other methods of tunneling. The stations on the subway line are the expensive part. And so even if you have to make your tunnel bigger, the idea is that you'll probably still come out ahead in the end because stations are just that expensive. As I mentioned at the beginning of the video, there are actually two lines being talked about here, with splits at the end out on two different branches, so that you have a central core with two branches on either side. And the interesting thing is the design of Barcelona Line 9 really lends itself to this. As we've talked about in previous videos, when you want to have branches with high frequency trains operating out onto the branches and back in from them, you really want flyovers to prevent traffic jams. And usually this would require you to build a complicated above or even below ground structure to allow trains going out on one direction to flyover trains coming in from another. But because the tracks are stacked with the Barcelona Line 9 approach, having branches is really easy because you kind of get the flyovers for free. The tunnel simply splits so that trains going in one direction on the top level split into two different tracks, and trains going in the other direction on the other level split into two different tracks, and then the tracks simply wrap down to the same level. It's really elegant. The amazing thing about the whole single bore station within the bore single access shaft lots of elevators approach is that it's so interesting that what would be otherwise interesting elements of Barcelona Line 9 are things I haven't even mentioned. For example, it doesn't just use single bores, parts of Line 9 are elevated and parts use more traditional construction methods. The trains also use overhead line electrification, which used to be less common with metro systems and it's fully automated with platform screen doors. The wonderful thing about this is it allows you to actually see all of that fascinating infrastructure sitting at the front of the train. But hold up, if this is such an incredible way of building Metro Line, why don't we hear more about it? Yes, Bard is planning on doing something similar, though for strange reasons. Since the whole point of Barcelona Line 9 was this was a construction method that you would use in a dense old European city center where other methods of construction were not possible, but other than that you really don't hear that much about doing this whole station within the tunnel bore approach. And that's because lines 9 and 10 have had a ton of problems. For one, construction on the lines still isn't done despite starting in 2002, 20 years ago. And so currently Line 9 and Line 10 for that matter are actually two separate lines where you have a northern section and a southern section with the connecting tunnels between them not complete. And it currently sounds like the full line might not actually be completed for a couple more years, if it doesn't get delayed further to 2030 and potentially beyond. Because of this, the southern portion of Line 10 can't even go as far as the southern portion of Line 9 because the single bore continues on without a ramp track to connect the two levels, meaning trains have to operate relatively infrequently. Part of the issue here is soil, as I mentioned before. The project used an earth pressure balanced tunnel boring machine, which is actually not all that uncommon these days, and is used when soil needs to be kept under pressure to prevent it from collapsing onto itself. But that kind of risk from poor soil, which can lead to ground settlement and even sinkholes, which you really don't want under your historic European city center, is made even worse when you use a gigantic tunnel boring machine, which has a disproportionately large tunnel size compared to two smaller machines. Another problem is that with the nature of the way TBMs work and with being deep underground, is that if you have a problem, you're kind of in trouble. You kind of have to keep going, or just go super slowly, since actually accessing the tunnel boring machine is incredibly hard if it's deep underground, and you can't exactly just do a U-turn. At the same time, with a larger tunnel boring machine, you're generally forced to go deeper than you would have to with smaller machines. As it turns out, Line 9 was actually going to be the longest underground metro line in Europe, at least when it started construction. But now it actually seems like that might not be the case anymore, since Line 15 in Paris is flying along, and it seems to be set to grab that record, and potentially the longest underground metro line in the world. It's mostly being constructed by more traditional methods as well. At the same time though, to be fair, tunneling is almost always super complicated. So for me, it's hard to tell if this is just a case of Line 9 and 10 getting really, really unlucky. We've really only seriously done this approach once. The shared tunnel thing is done in other places, but the station in the tunnel thing, that's quite novel. And while at the time, the size of the TBM used on the project seemed kind of gargantuan, it seems like over the 20 years that the project has been underway, we've gotten a lot better at tunneling, and larger TBMs seem to be the norm now. For example, Toronto is building the Scarborough subway extension with a large TBM that's similar in size to Barcelona Line 9's. Albeit in Toronto, we aren't building the stations within the bore. This is probably the case, because in Toronto, the trains are over 3 meters wide, while in Barcelona, on Line 9, trains are 2.7 meters wide and 90 meters long. Kind of similar to the Ontario Line. A little bit smaller. Now part of me hopes that this approach gets tried again, and maybe will be successful, and usher in a sea change in the way that we construct subways and metros. But in all honesty, the transit industry is pretty conservative, and people really don't like to hear that the project is over budget or delayed, so taking risks is not encouraged. But all of that just makes Barcelona Line 9 all the more special, because they were creative, and took risks. And that's super, super cool. A special thanks to Inijo and Powell for their footage on the ground from Barcelona that was used in this video.