At a meeting with other TransitMatters people, I had to explain various distinctions in what is called in American parlance regional rail or commuter rail. A few months ago I wrote about the distinction between S-Bahn and RegionalBahn, but made it clear that this distinction was about two different things: S-Bahns are shorter-distance and more urban than RegionalBahns, but they’re also more about service in a contiguous built-up area whereas RegionalBahns have the characteristics of interregional service. In this post I’d like to explore the different travel markets for regional rail not as a single spectrum between urban and long-range service, but rather as two distinct factors, one about urbanity or distance and one about whether the line connects independent centers (“interregional”) or a monocentric urban blob (“intraregional”).
This distinction represents a two-dimensional spectrum, but for simplicity, let’s start with a 2*2 table, so ubiquitous from the world of consulting:
|Connection \ Range||Short||Long|
|Intraregional||Urban rail, S-Bahn||Big-city suburban rail|
|Interregional||Polycentric regional rail||RegionalBahn|
The notions of mono- and polycentricity are relative. Downtown Providence, Newark, and San Jose all have around 60,000 jobs in 5 km^2. But Caltrain and the Providence Line are both firmly in the RegionalBahn category, the other end being Downtown San Francisco or Boston, 70-80 km away with 300,000-400,000 jobs in 5-6 km^2. Newark, in an essentially contiguous urban area with New York, 16 km from Midtown and its 1.2 million jobs in 6 km^2, is relatively weaker and does not fit into the interregional category; a New York-Newark line is an S-Bahn.
On the 2*2 table, the appellations “big-city” and “polycentric” are necessary. This is because longer-range rail lines are likelier to get out of the city and its immediate suburbs and connect to independent urban centers. Exceptions mostly concern the size of the primary urban cluster. If it is large, like New York, it can cast a shadow for tens of kilometers in each direction: commuter volumes are high from deep into Long Island, as far up the Northeast Corridor as Westport, as far up the Hudson as northern Westchester, and so on. In Paris, I wouldn’t be comfortable describing any of the RER and Transilien lines as RegionalBahn. In London, the closest independent cities of reasonable size are Cambridge, Brighton, Oxford, and Portsmouth, the first two about 80 km away and the last two about 100.
Tokyo, about as big as New York and London combined, casts an even longer shadow. In my post on S-Bahns and RegionalBahns I called some of its outer regional rail branches RegionalBahn, giving the examples like the Chuo Line past Tachikawa. But even that line is not really interregional in any meaningful way. It stays within the Tokyo prefecture as far as Takao, 53 km from Tokyo Station, and commuter service continues until Otsuki at kp 88, but everything along the line is bedroom communities for Tokyo or outright rural. The branching and short-turns at Tachikawa mean that the Chuo Line through Tachikawa is a long S-Bahn, and past Tachikawa is really a suburban commuter line too long to be an S-Bahn but too monocentric and peaky to be Regionalbahn (the peak-to-base frequency ratio is about 2:1, whereas German RegionalBahn is more commonly 1:1).
At the other end, we can have regional rail that is short-range but connects two distinct centers. This occurs when relatively small cities are in proximity to each other. In a modern first-world economy, these cities would form a polycentric region, like the Rhine-Ruhr or Randstad. Smaller regions with these characteristics include the Research Triangle, where relatively equal-size Raleigh and Durham are 40 rail kilometers apart, and Nord, where Lille is 30-50 km from cities like Douai and Valenciennes. This may even occur in a region with a strong primary center, if the secondary center is strong enough, as is the case for Winterthur, 28 km from Zurich, which has Switzerland’s fourth highest rail ridership.
Size is measured in kilometers, not people. Stockholm is a medium-size city region, but Stockholm-Uppsala is firmly within RegionalBahn territory, as the two cities are 66 km apart. Randstad’s major cities are all closer to each other – Amsterdam-Rotterdam is about 60 km – and that’s a region of 8 million, not 3 million like Stockholm and the remainder of Uppland and Södermanland.
The issue of frequency
The importance of the 2*2 table is that distance and urban contiguity have opposite effects on frequency: high frequency is more important on short lines than on long lines, and matching off-peak frequency to peak frequency is more important on interregional than intraregional lines.
Jarrett Walker likes to say that frequency is freedom, but what frequency counts as freedom depends on how long passengers are expected to travel on the line. Frequency matters insofar as it affects door-to-door travel time including wait time, so it really ought to be measured as a fraction of in-vehicle travel time rather than as an absolute number. An urban bus with an average passenger trip time of 15 minutes should run every 5 minutes or not much longer; if it runs every half hour, it might as well not exist, unless it exists for timed connections to longer-range destinations. But an intercity rail line where major cities are 2 hours apart can easily run every half hour or even every hour.
The effect of regional contiguity is more subtle. The issue here is that an intraregional line is likely to be used mostly by commuters at the less dense end. The effect of distance can obscure this, but within a large urban area, a 45-minute train will be full of commuters traveling to the primary city in the morning and back to the suburbs in the afternoon or evening; the same train between two distinct cities, like Boston and Providence, will not have so many commuters. In contrast, the same 45-minute trip will get much more reverse-commute travel and slightly more non-commute travel if it connects two distinct cities, because the secondary city is likelier to have destinations that attract travelers.
In no case are the extreme peak-to-base ratios of American commuter lines justifiable. Lines with tidal commuter flows can run 2:1 peak-to-base ratios, as is common in Tokyo, but much larger ratios waste capacity. The marginal cost of service between the morning and afternoon peaks is so low until it matches peak service that having less midday than peak service at all is only justifiable in very peaky environments. The 45-minute suburbs of New York, Tokyo, and other huge cities can all live with a 2:1 ratio, but other lines should have lower ratios, and interregional lines should have a 1:1 ratio.
The implication is that just as peak-to-base ratios going as high as 2:1 are acceptable for long-range intraregional lines, short-range interregional lines must run a constant, high frequency all day. I would groan at the thought of even half-hourly frequency on a 40-km interregional line; the worst I’m comfortable with is 15-20 minutes all day. Of note, such lines are necessarily pretty fast, since by assumption they make few intermediate stops to speed up travel between the two main cities – if there are significant cities in the middle then the lines connect even shorter-range cities and should be even more frequent.
Urban, suburban, intercity
Individual lines may have the characteristics of multiple variants of regional rail. They pass through urban neighborhoods on their way to outlying areas, which may be suburbs or independent cities; they may also pass through multiple kinds of independent areas.
In practice, in big cities this leads to three tiers on the same line: urban at the inner end, suburban at the middle end, interregional at the outer end. Inversions, in which there are independent cities and then suburbs, are possible but extremely rare – I can’t think of any in Paris, London, or New York, and arguably only three in Tokyo (Chiba, Saitama, Yokohama); fundamentally, if there are suburbs of the primary city beyond your municipality, then your municipality is likely to itself be popular as a suburb of the primary city.
That regional lines have these three tiers of demand type does not mean that every single regional line does. Some lines don’t reach any significant independent city. Some don’t usefully serve close-in urban areas – for example, the Providence Line barely serves anything urban, since the stop spacing is wide in order to speed up travel to high-demand suburbs and to Providence and the closest-in urban neighborhoods have Orange Line subway service. In rare cases, the suburban tier may be skipped, because there just isn’t much tidal suburban commuter ridership; in Boston, the Newburyport Line is an example, since its inner area has unbroken working-class urban development almost all the way to Salem, and then there’s almost nothing between Salem and Newburyport.
This does not mean that suburbs are always in between urban areas and independent cities – this is just a specific feature of large metropolitan areas. In smaller ones, the middle tier between urban and long-range interregional service is occupied by short-range interregional service rather than suburban commuter rail. Skipping the suburban tier, which is rare enough in large cities that in the cities I think about most often the only example I can come up with is the Newburyport Line, is thus completely normal in smaller cities.
There are common best practices for commuter rail: electrification, level boarding, frequent clockface schedules, timed transfers, fare integration, proof of payment fare collection.
However, high frequency means different things on lines of different characteristics. An interregional line should be running consistent all-day frequency, and if it is long enough could make do with half-hourly trains with timed connections to suburban buses; an urban line should be running every few minutes as if it were a metro line. Regional rail lines with characteristics off the main diagonal of the S-Bahn to RegionalBahn spectrum have different needs – suburban lines can have high peak frequency to reduce road congestion, although they should still have useful off-peak frequency; short-range interregional lines should run every 10-20 minutes all day.
The distinctions between intraregional and interregional lines and between short- and long-range lines may also affect other aspects of planning: station spacing, connections to local surface transit, connections at the city center end, through-running, etc. Even when the best industry practices are the same in all cases, the relative importance of different aspects may change, which changes what is worth spending the most money on.
Since an individual line can serve multiple markets on its way from city center to a faraway outlying terminal, it may be useful to set up a timetable that works for all of these markets and their differing needs. For example, urban lines need higher frequency than suburban and interregional ones, so a regional line with significant urban service should either branch or run short-turn trains to beef up short-range frequency. If there is a suburban area in the middle with demand for high peak frequency but also a secondary city at the outer end, it may be useful to give the entire line high all-day frequency, overserving the line off-peak just because the cost of service is low.
Ultimately, regional rail is about using mainline rail to fulfill multiple functions; understanding how these functions works is critical for good public transportation.