Lines on Maps

It’s been an exciting week for transport news:

That last news is about a new agreement between the City of Stockholm and its suburbs Nacka and Värmdö to carry out a pre-study of an extension of the tunnelbana, or underground, to the fastest-growing corner of the Stockholm metro area, currently served by extensive commuter buses and a very limited, aging commuter rail line, which due to long sections of single track can run at most one train per 20 minutes. This announcement kicks off a new round of enthusiasm from politicians, media, and the general public, continuing several decades of experts and public transport enthusiasts alike (the line is rather fuzzy) drawing lines on maps of exactly where this new extension could go on its way to the southeastern corner of the region.

The new twist today is a proposal by construction firms Skanska and Sweco (in Swedish) to reduce costs by combining the new underground line with the proposed Eastern Link motorway, since both require a new tunnel under Stockholm’s inner harbor.

While the new proposal, shown above left, is intriguing for its efficiency, it’s also noteworthy in its omission of any stop on Södermalm (the south island of the central city) or Hammarby Sjöstad. The only stop between the core and Nacka is at Skansen, a tourist-only destination that is already served by Stockholm’s recently updated #7 tram line. It thus loses two potential benefits: trips from the southeast suburbs to Södermalm, and trips across Södermalm.

The first, trips to Södermalm, are clearly better served by SL’s preferred alignment, shown above right. But what’s interesting is that even that alignment doesn’t do much to help intra-Södermalm trips. This has been an underserved dimension since the tunnelbana was first designed. Characteristic of most radial transport systems, all three lines — as well as the commuter rail line — would be oriented toward the city center, north of the Mälar-Baltic channel (see this map of Stockholm’s inner city).

The origin of the problem is that Stockholm’s tunnelbana has from the beginning been designed as a radial system with elements of spiral curves. In the entire system there are only two intersections between two different lines: T-Centralen, which is truly the city center; and Fridhemsplan, where the Green and Blue lines cross. Fridhemsplan is an excellent example of how the configuration can facilitate trips that don’t go through the city center, shortening those trips and alleviating congestion in T-Centralen. A better example might be the DC Metro, where the three central alignments cross at different stations, creating a central triangle.

Can the tunnelbana configuration in Stockholm be changed to create more transfer points? Not without adding significant distance and tight curves to this new extension of the Blue line.

What about a surface tram line? The planned extension of Tvärbanan will end up at Slussen and head north, just like the tunnelbana. How about buses? Unfortunately, Södermalm’s street grid and turn restrictions make cross-island bus lines extraordinarily circuitous, best exemplified by Bus 66 (see again the same map linked above).

It seems like for the foreseeable future, the best way for Södermalm’s artists, hipsters, youngsters, and the rest of the stereotypical population will be to walk, bicycle, or transfer on buses to get around.

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