Most urban rail networks consist predominantly of radial lines, connecting city center with outlying areas. However, a fair minority attempt a different typology, reminiscent of a grid. This does not work so well in practice, and I want to bring it up in the context of both my current domicile and my previous one, which have non-radial metro systems, for different reasons. The problem with non-radial metro networks is that many trips require multiple transfers, and even single-transfer trips often place the connection far out of the way. The average effective speed in such cases is often lower than that of a slow bike.
Berlin’s S-Bahn network is fully radial, consisting of two radial trunks and the Ringbahn. However, its U-Bahn network fails to be radial in three distinct ways. First, the two north-south trunks, U6 and U8, are parallel. Second, the east-west U5 only goes from Alexanderplatz eastward, although an under-construction extension will extend it slightly to the west and connect with U6. And third, the postwar lines were constructed in a Cold War context around a temporary city center.
The problem can be summarized with the following map, in which the green dot is where I live and the three red dots are places I’ve gone to recently:
A larger 13 MB image can be seen here.
My trips to the two southern red dots, both gaming events, are not too onerous, using the Ringbahn. But my trip to the northern red dot isn’t so easy; besides being circuitous, the Ringbahn is currently shut down for maintenance for a segment in between. Now, there’s redundancy, I can still get there, but it requires a three-seat ride involving U7, U8, and U2, and what’s more, the U8/U2 transfer at Alexanderplatz is long.
The route U7 takes is a Cold War relic. During the division of Berlin, city center was in East Berlin, forcing West Berlin to build a new city center, currently called City West. East Berlin got the S-Bahn, which West Berliners eventually began to boycott even when it served the West, and West Berlin got nearly the entire U-Bahn, with the exception of just U5 and the eastern parts of U2. U6 and U8 served West Berlin going from the north to the south without stopping in the Walled center. To provide entirely Western routes, West Berlin built two new lines, U7 and U9.
The decisions made about U7 and U9 routing were then about the context of the Wall. U9 is a straight north-south trunk passing through City West, with connections to every West Berlin U-Bahn line with the exception of the low-ridership U4 stub. U7, originally part of the U6 mainline south of their junction, was extended northwest to become a new trunk line, and it too is designed to connect to the U-Bahn network of West Berlin alone rather than to the combined one. Thus, there is a reasonable U7-U1 connection at Möckernbrücke, but instead of continuing due west to connect with U2, U7 detours southwest and only connects with U2 at Bismarck Strasse, too far to the west to be of use for people connecting between the two lines’ eastern legs. In the Cold War, this worked, in the sense that the parts of U2 on the Western side of the Wall are either still convenient with the Bismarck Strasse transfer or very close to U1. In the context of reunification, this doesn’t work so well.
Here is a map of passenger rail traffic by interstation segment:
Note that U9 goes strong throughout its run; while the area it serves is no longer the CBD, it is still a strong high-end retail center, defined around Kurfürstendamm. However, U7’s traffic actually peaks in Neukölln and Kreuzberg and is lower west of the junction with U6 at Mehringdamm. Moreover, coming from Spandau at the other end, U7 loses traffic as it crosses the Ringbahn (as do many lines coming from the west) rather than gaining it. The shift in the center of the city has rendered U7 a mixed radial-circumferential line.
Like Berlin, Paris has a radial mainline rail network and a more complicated dedicated urban rail system.
The Metro defies easy categorization. It has the characteristics of a grid, with Lines 1, 3, 8, 9, and 10 running east-west, and Lines 4, 5, 7, 12, and 13 running north-south. It also has the characteristics of two overlain radial networks, one consisting of Lines 1, 4, 7, 11, and 14 around Chatelet, and one consisting of Lines 3, 7, 8, 9, 12, 13, and 14 around the Opera and Saint-Lazare. Some support for the latter view is that the weakest lines are M5 and M10, passing respectively too far east and south of city center.
Out of 14 Paris Metro lines, only two can connect to every other line in the network, M4 and M9, and the latter has an unadvertised connection from Saint-Augustin to Saint-Lazare, without which no transfer to M12 or M14 is possible.
This is not quite as dire as it may seem at first glance. So many stations are transfers, and stations are spaced so close together, that even though M1 runs parallel to M3 and M10, nearly all intra muros M1 stations still have two-seat rides to M3 and M10 via a different line.
Nevertheless, some parts of the city are poorly connected as a result. Eastern Paris’s connections to the Left Bank are not good. Some are more circuitous than they need to be, such as between a dorm I stayed at near Porte de Vincennes in 2010 for a conference and the site of the conference itself at Jussieu. Some are three-seat rides. Most involve changing trains at Chatelet, which adds 5-7 minutes to the trip in walking time alone.
In the case of Berlin, explaining how it got this way requires going into Cold War history. In that of Paris, a city with continuous development, this is just a matter of uncoordinated layers of planning. The plan from the 1890s produced M1-6, shaped as a hex symbol inside a circle; the lack of connection between M1 and M3 was not thought a problem then, and the remaining lack of connectivity if one originates in the suburbs was never a planning priority either.
There is another way
Paris may have Europe’s largest rail network, and Berlin may have the fourth largest, but they are in this sense atypical. London’s radial Underground network provides better connectivity, and the radial typology is increasingly dominant as Chinese metro networks follow the Soviet model, which is even more strictly radial than the British one.
The distance between my apartment and the northernmost red dot on the Berlin map is 8 kilometers. I looked at similar 8-kilometer trips within Inner London, picking two different starting points at Brixton and Bromley-by-Bow. Each 8-kilometer trip passing through or near Central London had a viable one- or two-seat ride, with the exception of Brixton-Canary Wharf, which is a three-seat ride with a cross-platform interchange at Stockwell.
There are a lot of defects in the London Underground network, and the two starting points I picked are somewhat cherrypicked to avoid them. Every pair of London Underground main lines intersects with a transfer, except the Metropolitan line and the Charing Cross branch of the Northern line, which have a missed connection at Euston and Euston Square; my two starting points are on neither of these two lines. Moreover, going northwest, there are some suburban missed connections between branches. This is on top of serious problems with capacity coming from the trains’ small size.
However, it’s notable that nobody is reproducing the small profile of the Tube networks. What cities around the world are reproducing is the radial network design, in which most trips within the urban core are reasonably direct. Going forward, Paris may even consider building connections between Metro lines to make its network more radial, for example extending M11 to the west. Berlin, likewise, should look to invest in radial S-Bahn trunks, following the busiest corridors connecting more areas to and beyond Mitte, where it’s already building extensive office space.