I made an off-hand remark about subway-surface systems, called Stadtbahn in German (as is, confusingly, the fully grade-separated east-west Berlin S-Bahn line), regarding a small three-line single-tail network that Brooklyn could build. I also talked about it in a little more detail last year. I want to go more deeply into this now. It’s a public transportation typology that’s almost nonexistent outside Germany and Belgium; Tel Aviv is building one line, and the US has three but two of those are from more than 100 years ago. But there are interesting examples of good places to use this technology elsewhere, especially elsewhere in Europe.
What is the Stadtbahn?
The Stadtbahn (“city rail”), or the subway-surface line in US usage, is an urban line running light rail vehicles, with grade separation in city center and street running outside city center. All examples I know of are in fact underground in city center, but elevated lines or lines running in private rights-of-way could qualify too, and in Cologne, there’s a semi-example over a bridge dropping to the surface at both ends.
It’s best illustrated as a 2*2 grid:
|Slow in center||Fast in center|
|Slow in outlying areas||Tramway||Stadtbahn|
|Fast in outlying areas||Tram-train||Rapid transit|
The terms fast and slow are relative to general traffic, so a mixed-traffic bus in a low-density city that averages 30 km/h is slow whereas the Paris Métro, which averages 25 km/h, is fast; the speed in km/h may be higher on the bus, but the speed in destinations accessed per hour is incomparably higher on the Métro.
The tram-train is confusingly also called Stadtbahn in Germany, for example in Karlsruhe; this is nearly every light rail built in North America. It is not the topic of this post.
What is the purpose of the Stadtbahn?
Historically, Stadtbahn systems evolved out of pure surface tramways. City center congestion made the streetcars too slow, so transit agencies put the most congested segments underground. This goes back to Boston in 1897 with the Tremont Street Subway and Philadelphia in 1906 with the Subway-Surface Lines. The contrast both in that era and in the era of Stadtbahn construction in Germany from the 1960s to the 80s is with pure subways, which are faster but cost more because the entire route must be underground.
Stadtbahns always employ surface branching. This is for two reasons. First, there’s more capacity underground than on the surface, so the higher-capacity rapid transit segment branches to multiple lower-capacity tramways to permit high throughput. And second, there’s generally less demand on the outer segments than in the center – lines with very strong demand all the way tend to turn into full subways.
This is therefore especially useful for cities that are not huge. In a city the size of Cologne or Stuttgart or Hanover, there isn’t and will never be demand for a rapid transit system with good citywide coverage. Instead, there is something like a sector principle. For example, in Cologne, the Deutz side of the city, on the right bank of the Rhine, has service to city center on the S-Bahn, on tramway lines over the Deutzer Bridge branching on the surface, and on tramway lines over the Mülheimer and Severin Bridges feeding into the north-south ring Stadtbahn. Smaller cities have simpler systems – Hanover for has three underground trunk lines meeting at one central station, and Dortmund has three meeting in a Soviet triangle. This maintains good coverage even without the budget for many rapid transit lines.
Where are Stadtbahns appropriate?
Cities should consider this technology in the following cases:
- The city should not be too big. Tel Aviv is too big for this, and people in Israel are starting to recognize this fact and, in addition to the under-construction three-line Stadtbahn system are proposing a larger-scale three-line fully grade-separated metro system. If the city is big enough, then a full metro system is justified.
- There should be a definitive city center for substantial traffic to funnel to. The purpose of the Stadtbahn is to have comparable throughput to that of a metro, albeit with shorter trains.
- There should be wide swaths of sectors of the city where having multiple parallel lines is valuable. This, for example, is the case in cities that are not exceptionally dense and cannot expect transit-oriented development to completely saturate one metro corridor.
- The street network should not be too gridded, because then the sector-based branching is more awkward, and the combination of rapid transit to city center and a surface transit grid can be powerful, as in Toronto.
- There should be too much city center congestion for a pure surface system to work, for example if most streets are very narrow and traffic funnels to the few streets that can use
These circumstances are all common to German urbanism: city centers here are strong, but residential density peaks at 15,000/km^2 or thereabouts in near-center neighborhoods and drops to 3,000-6,000/km^2 farther out. Moreover, Germany lacks huge cities, and of the largest four urban cores – Berlin, Hamburg, Munich, Frankfurt – three have full rapid transit systems. Finally, grids are absent here except at very small scale, as in Mannheim.
However, these are not unique features to Germany. They’re common around Europe. European cities are not very big, and the only ones that can genuinely fill any subway line with transit-oriented development are a handful of very big, very rich ones like London and Paris. Even Stockholm and Munich have to be parsimonious; they have have full metro systems with branching.
The French way of building rapid transit does not employ the Stadtbahn, and perhaps it should. In a city the size of Bordeaux or Nice, putting a tramway underground in city center and then constructing new branches to expand access would improve coverage a lot.
This is likely also the case in Italian cities below the size class of Milan or Rome. Many of these cities are centered around Renaissance cores with very narrow streets, which are nonetheless auto-centric with impossibly narrow sidewalks, Italy having nearly the highest car ownership in Europe. Finding one to three good corridors for a subway and then constructing tramways funneling into them would do a lot to speed up public transit in those cities. Bologna, for example, is planning a pure surface tramway, but grade-separated construction in the historic center would permit trams to have decent coverage there without having to slow down to walking speed.
Are there good examples outside Europe?
Yes! From my original post from 2016, here is one proposal for New York:
The B41 could be a tramway going between City Hall and Kings Plaza, using two dedicated lanes of the Brooklyn Bridge. In that case, the line would effectively act as subway-surface, or more accurately elevated-surface: a surface segment in Brooklyn, a grade-separated segment between Manhattan and Brooklyn. Subway-surface lines should branch, as all current examples do (e.g. Boston Green Line, Muni Metro, Frankfurt U-Bahn), because the subway component has much higher capacity than the surface components. This suggests one or two additional routes in Brooklyn, which do not have strong buses, but may turn into strong tramways because of the fast connection across the river to Manhattan. The first is toward Red Hook, which is not served by the subway and cut off from the rest of the city by the Gowanus Expressway. Unfortunately, there is no really strong corridor for it – the streets are not very wide, and the best for intermediate ridership in Cobble Hill and Carroll Gardens require additional twists to get into the core of Red Hook. Court Street might be the best compromise, but is annoyingly a block away from the F/G trains, almost but not quite meeting for a transfer. The second possible route is along Flushing Avenue toward the Navy Yard; it’s not a strong bus by itself, but the possibility of direct service to Manhattan, if a Flatbush tramway preexists, may justify it.
Note that this proposal is opportunistic: Brooklyn Bridge just happens to be there and point in the right direction for at least one strong surface route in Brooklyn, and conversely would connect too awkwardly to the subway. In a city the size of New York, Stadtbahn lines must be opportunistic – if the region intentionally builds new river crossings then they must carry the highest-capacity mode of transportation, which is rapid transit, not a light rail variant.
American cities smaller than New York are often very big by European standards, but also very decentralized. This hurts the Stadtbahn as a mode – it really only works for a monocentric city, because if there are multiple centers, then all but the primary one get slow transit. The Rhine-Ruhr notably uses the S-Bahn, which is rapid transit, to connect its various cities, and only run Stadtbahn service internally to each center, like Cologne or Dortmund.
There are a number of places in the United States where burying a light rail line in city center is advisable, but this is for the most part conversion of a tram-train to rapid transit, for examples in Portland and Dallas. The only example that come to mind of a decent Stadtbahn in the US that doesn’t already exist is Pittsburgh, converting the BRT system to rail.
Outside the United States, I get less certain. Canada is bad geography for a Stadtbahn because of its use of grid networks; Ottawa may be good for a Stadtbahn using the Confederation Line tunnel, but that’s probably it. Australia may be better, combining decently strong city centers with very low residential density; transit-oriented development potential there is very high, but it could plausibly come around multiple distinct corridors as well as regional rail stations. Melbourne’s tramways thus may be a candidate for Stadtbahn conversion.
In both East Asia and in the developing world, it’s likely best to just build full metros. East Asian cities are big and have high rates of housing construction (except Hong Kong). I can see a Stadtbahn succeed in Taichung, extending the under-construction Green Line on the surface and building intersecting lines, but that’s probably it. Kaohsiung already has a (very underused) subway, what I think is Daejeon’s best next corridor on top of Line 1 and the planned Line 2 is unusually bad for a Stadtbahn because the streets are too gridded west of the center, Daegu is too gridded as well.
A similar set of drawbacks is also true for the developing world. The urban population of the developing world tends to cluster in huge cities. Moreover, these cities tend to have high residential density but low city center job concentration; the Addis Ababa light rail is bad at serving people’s work trips because so few people work in the center. Finally, the developing world has high rates of increase in urbanization, which make future-proofing systems with higher capacity more valuable.