I wrote a long thread about regional rail and population density, and I’d like to explain more and give more context. The upshot is that higher population density makes it easier to run a rail network, but the effects are most visible for regional rail, rather than either urban rail or high-speed intercity rail. This is visible in Europe when one compares the networks in high-density Germany and low-density Sweden, and has implications elsewhere, for example in North America. I stress that high-speed rail is not primarily affected by background density, but only by the populations of cities within a certain range, and thus France, which has one of Western Europe’s lowest densities, manages to have high per-capita ridership on the TGV. However, the density of a regional mesh comes from background density, which is absent in such countries as France, Sweden, and Spain.
What is density?
Population density is population divided by area. This post is concerned with overall density at the level of an entire country or region, rather than the more granular level of the built-up urban area of a single city. What this means is that density is in large part a measurement of how close cities are to one another. In a high-density area like western Germany, Northern Italy south of the Alps, England, or the Low Countries, cities are spaced very close together, and thus people live at densities surpassing 300/km^2. In contrast, low-density areas have isolated cities, like Sweden, Australia, Canada, or the Western United States.
For example, take Stockholm. The region has about 2.5 million people, and has a strong urban and suburban rail network. However, there just aren’t a lot of cities near Stockholm. The nearest million-plus metro areas are Oslo, Gothenburg, and Helsinki, all about 400 km away, none much bigger than 1 million; the nearest 2 million-plus metro area is Copenhagen, 520 km away. The region I use as an example of German polycentrism, Rhine-Neckar, is about the same size as Stockholm, and has a good deal more suburban sprawl and car usage. The nearest million-plus region to Mannheim is Karlsruhe, 55 km away; it is a separate metropolitan area even though the Rhine-Neckar S-Bahn does have an hourly train to Karlsruhe. Frankfurt is 70 km away. A 400 km radius from Mannheim covers nearly the entirety of Germany, Switzerland, and the Low Countries; it reaches into Ile-de-France and into suburbs that share a border with Amsterdam. A 520 km radius covers Paris, Berlin, Hamburg, Milan, and Prague, and reaches close to Vienna.
Density and regional rail
Kaiserslautern is a town of 100,000 people, served by the Rhine-Neckar S-Bahn every half hour even though it is not normally seen as part of the Rhine-Neckar region. It has, in addition to the east-west S-Bahn, independent regional lines reaching north and south. When I visited two years ago, I saw these lines pulse while waiting for my delayed TGV back home to Paris.
This is viable because there are towns ringing Kaiserslautern, close enough that a low-speed regional train could connect them, with their own town centers such that there is a structure of density around their train stations. This in turn exists because the overall population density in Germany is high, even in Rhineland-Pfalz, which at 206/km^2 is slightly below the German average. The alternative structure to that of Germany would have fewer, larger cities – but that structure lends itself well to regional rail too, just with fewer, thicker lines running more frequently. If those smaller towns around Kaiserslautern did not exist but people instead lived in and right around Kaiserslautern, then it would be a city of about 400,000, and likewise Mainz might have 500,000 and the built-up area of Mannheim would have more people in Mannheim itself and in Ludwigshafen, and then there would be enough demand for a regional train every 10-20 minutes and not just every half hour.
I bring up Sweden as a low-density contrast, precisely because Sweden has generally well-run public transport. Stockholm County’s per capita rail ridership is higher than that of any metropolitan area of Germany except maybe Berlin and Munich. Regional rail ridership in and around Stockholm is rising thanks to the opening of Citybanan. Moreover, peripheral regions follow good practices like integrated intermodal ticketing and timed transfers. And yet, the accretion of a mesh of regional lines doesn’t really exist in Sweden. When I visited Växjö, which is not on the main intercity line out of Stockholm, I had a timed connection at Alvesta, but the timetable there and at Växjö looked sporadic. Växjö itself is on a spur for the network, but poking around the Krösatågen system it doesn’t look like an integrated timed transfer system, or if it is then Alvesta is not a knot. I was told in the replies on Twitter that Norrbotten/Västerbotten has an integrated network, but it runs every 2 hours and one doesn’t really string regional rail lines together to form longer lines the way one does in Germany.
Integrated regional networks
The integrated timed transfer concept, perfected in Switzerland, is ideal for regional and intercity networks that form meshes, and those in turn require high population density. With these meshes, regional rail networks overlap, underlaying an intercity network: already one can get between Frankfurt and Stuttgart purely on lines that are branded as S-Bahn, S-Bahn-like, or Stadtbahn, and if one includes RegionalBahn lines without such branding, the network is nationally connected. Even in Bavaria, a state with lower density than the German average, nearly all lines have at least hourly service, and those form a connected network.
It’s perhaps not surprising that Italy, which has high density especially when one excludes unpopulated alpine areas, is adopting German norms for its regional rail. As in Germany, this originates in urban networks, in Italy’s case that of Milan, but Trenord operates trains throughout Lombardy, most of whose population is not the built-up area of Milan, and even lines that don’t touch Milan run hourly, like Brescia-Parma. Italy is not unusual within Southern Europe in looking up to Germany; it’s only unusual in having enough population density for such a network..
Once the network is in place, it is obligatory to run it as an integrated timed transfer system. Otherwise, the connections take too long, and people choose to drive. This in turn means setting up knots at regular intervals, every 30 minutes for a mixed hourly and half-hourly system, and investing in infrastructure to shorten trip times so that major cities can be knots.
The concept of the knot is not just about regional service – high-speed rail can make use of knots as well. Germany has some low-hanging fruit from better operations and under-construction lines that would enable regularly spaced knots such as Frankfurt, then Mannheim, then Stuttgart, and far to the north Hanover and then Bielefeld. The difference is that Germany’s ideal high-speed rail network has around 20 knots and its existing regional rail network has about as many in Hesse alone. Nor can regional rail networks expect to get away with just building strong lines and spamming frequency on those, as the Shinkansen does – regional rail uses legacy alignments to work, generating value even out of lines that can only support an hourly train, whereas high-speed lines need more than that to be profitable.
Globally, the lowest-hanging fruit for such a system is in the Northeastern United States, followed by China and India. Population density in the Northeast is high, and cities have intact cores near their historic train stations. There is no excuse not to have a network of regional lines running at a minimum every 30 minutes from Portland down to Northern Virginia and inland to Albany and Harrisburg.
A few modifications to the basic Swiss system are needed to take into account the fact that the Northeast Corridor, run at high speeds, would fill a train every 5 minutes all day, and the core regional lines through New York could as well. But regional rail is not a country bumpkin mode of transportation; it works fine within 100 km of Frankfurt or Milan, and should work equally well near New York. If anything, a giant city nearby makes it easier to support high frequency – in addition to internal travel within the regional system, there are people interested in traveling to the metropole helping fill trains.
What about low-density places?
Low-density places absolutely can support good rail transport. But it doesn’t look like the German mesh. Two important features differ:
- It is not possible to cobble together a passable intercity rail network from regional express lines and upgrade it incrementally. Intercity lines run almost exclusively intercity traffic. This tilts countries toward the use of high-speed rail, including not just France but also Spain and now Sweden. This does not mean high-density countries can’t or shouldn’t build high-speed rail – they do successfully in Asia, Italy has a decent network, Britain has high-speed rail plans, and Germany is slowly building a good network. It just means that high-density countries can get away with avoiding building high-speed rail for longer.
- The connections between regional and intercity lines are simpler. Different regions’ suburban networks do not connect, and can be planned separately, for example by state-level authorities in Australia or provincial ones in Canada. These networks are dendritic: intercity lines connect to regional lines, and regional lines branch as they leave city center. Lines that do not enter the primary city center are usually weaker, since it’s unlikely that there are enough strong secondary centers at the right places that a line could serve them well without passing through the primary center.
In extreme cases, no long-distance rail is viable at all. Australia is a borderline case for Brisbane-Sydney-Melbourne high-speed rail – I think it’s viable but only based on projections of future population and economic growth. But Perth and Adelaide are lost causes. In the United States, railfans draw nationally-connected proposals, but in the Interior West the cities are simply too far apart, and there is no chance for a train to usefully serve Denver or Salt Lake City unless cars are banned. Connecting California and the Pacific Northwest would be on the edge of viable if the topography were flat, but it isn’t and therefore such a connection, too, is a waste of money in the economic conditions of the early 21st century.
Note that even then, cities can have suburban rail networks – Perth and Adelaide both have these, and their modal splits are about on a par with those of secondary French cities like Nice and Bordeaux or secondary American transit cities like Boston and Chicago. Denver is building up a light rail and a commuter rail network and one day these networks may even get ridership. The difference between the case of Perth or Denver and that of a German city is that Perth and Denver can rest assured their regional rail alignments will never be needed for intercity rail.
In less extreme cases, intercity trains are viable, and can still run together with regional trains on the same tracks. California is one such example. Its population density and topography is such that planning regional rail around the Bay Area and in Los Angeles can be kept separate, and the only place where intercity and regional trains could work together as in Germany is the Los Angeles-San Diego corridor. Blended planning with timed overtakes is still recommended on the Peninsula, but it’s telling that at no point have Bay Area-based reformers proposed a knot system for the region.
Those less extreme low-density cases are the norm, in a way. They include the Midwestern and Southern US, the Quebec-Ontario corridor, the Nordic countries, France, nearly all of Eastern Europe, and Southern Europe apart from Italy; this is most of the developed world already. In all of those places, regional rail is viable, as is intercity rail, but they connect in a dendritic and not meshlike way. Many of the innovations of Germany and its penumbra, such as the takt and the integrated intermodal plan, remain viable, and are used successfully in Sweden. But the exact form of regional rail one sees in Germany would not port.