Many cities have large commuter rail networks, which function as urban rapid transit and extend into the suburbs. They use mainline rail rather than separate subway tracks, but are identical in other respects to conventional metro systems: urban stop spacing, frequency, and fares are all within the range of metro systems.
The biggest systems are in Japan, where Tokyo and Osaka carry the vast majority of their public transport passengers on commuter trains and not metro trains. In Europe, the biggest system is the Paris RER, while in the German-speaking world all major cities have S-Bahn networks.
But while the concepts are all broadly similar – mainline trains serving the suburbs share tracks in the core, so as to provide metro-like frequency – the stopping patterns vary.
First, some regional rail systems run express trains, whereas others are all local. The Munich and Berlin S-Bahns only have local trains. In contrast, the Paris RER and the Tokyo commuter rail network combine local and express trains – sometimes on four tracks, and sometimes on two, using the schedule to avoid conflicts.
Usually, systems that run express trains are bigger than systems that do not, but there are exceptions: Copenhagen's S-Train has express trains on most branches, and the Zurich S-Bahn has express trains on some lines as well.
On systems that are not modernised, express trains are especially common. That’s because the traditional function of commuter rail is to connect the suburbs with city center at rush hour; local services, connecting the suburbs with each other and not just with the city, are less important.
As a result, American commuter lines, even ones with very little ridership by European or Asian standards, generally have express trains: each stopping pattern might only get 1 or 2 trains per hour at rush hour. So do a few European branches, for example some of the outer commuter lines in Paris, not connected to the RER. Since these lines carry few riders, the important distinction is between different local and express patterns on busy lines that run frequently all day.
There isn't much to say about local trains, which (mostly) stop at every station. Express trains are more complex, and there are two ways to run them: one common in Paris, the other common in Tokyo. The Parisian model is to have long central segments with only two tracks, on which every train makes every stop. (In London, Crossrail is planned to follow the same pattern.) Tokyo’s railways have four-track segments, and express trains skip some stops even in the core.
Tokyo-style express trains may skip fewer stops in the centre than in dormitory communities, but they still skip even some central areas. On the eight-track main between Tokyo and Shinagawa Stations on the Tokaido Line, for example, the local Yamanote and Keihin-Tohoku Lines make all four intermediate stops; but the express Tokaido Main Line and Yokosuka Lines only make one intermediate stop, at Shimbashi. Central Tokyo stretches roughly between Tokyo Station and Shimbashi, and there is one station between them, Yurakucho, with transfers to four Tokyo subway lines. But in the judgement of rail planners, it made sense to skip this station, and for express trains to serve just one in every so many stops on the inner part of the line.
In Paris, no such thing service exists: the central tunnels only have two tracks, so it is hard to arrange local and express trains on them. Even on the few segments of the central network that have four tracks, such as part of RER C, there is no stop skipping. The transport authorities judge it best to have every commuter train make every stop within the city proper, which extends about 5 km out of the center.
Conversely, in the suburbs, Paris does mix some local and express trains on two tracks: the RER B runs 12 trains per hour off-peak – just enough room for trains which run non-stop between Gare du Nord and Charles-de-Gaulle Airport, and some express trains in the southern suburbs.
The Parisian approach ensures that the RER can function as high-frequency trunk lines within the city proper. The RER A averages a stop per 2.5 km on the central trunk, and the RER B and C a stop every 1.2-1.3 km (the other two RER lines, the D and E, only make three city stops each). The Metro averages a stop every 500 meters, of course – but nonetheless, 1.2-1.3 km is well within the range for international metro systems, comparable to the spacing of stations on the London Underground. The central Crossrail trunk will average a stop every 1.6 km – wider than the Underground but not much more so.
In Tokyo, of course, the commuter rail frequency in the core is even higher, since the inner lines are all at least four-tracked. But farther out, there are express and local trains mixed on two tracks, with timed overtakes, using the legendary punctuality of Japanese railways to schedule trains to avoid conflicts. The result is that the express routes have quite wide stop spacing, which permits higher speeds, approaching 60 km/h on the Tokaido and Tohoku Main Lines.
A smaller city, with trunk lines not as full to capacity as in Tokyo, Paris, or London, could mix local and express trains even at rush hour. In Tokyo, local and express trains are mixed on some lines on the shoulders of rush hour (but not at rush hour, when trains arrive every 2 minutes); it is unclear what the absolute upper limit of this system would be, but it appears to be in the range of 15-20 trains per hour. In cities without Japanese punctuality, the limit is about 12 trains per hour: a local train and an express train each coming every 10 minutes, with an overtake every 6-8 stations.
Such cities have a choice. The Paris approach works very well for Paris, and the Tokyo approach works very well for Tokyo. There is always a tradeoff in mass transit between narrow stop spacing for service to more places, and wide stop spacing for higher average speed.
The two different approaches for commuter rail express stopping patterns display a related tradeoff, between higher frequency to all stations and higher average speed at express stations. Which of the two approaches is better depends on local factors. These include city size and density (more sprawl encourages the faster Tokyo approach, more density encourages the more frequent Paris approach); punctuality (better punctuality makes mixing local and express trains on two tracks easier); and how important it is that suburban commuters be able to reach every urban station, rather than just a few major stations.
There is no inherent better choice. The tradeoff is not that one option is more beneficial but more expensive, but rather that the two options have different benefit levels, depending on local conditions.
Those conditions can vary widely between cities, even in the same country. A smaller French or British city might find that its home and job distribution makes the Tokyo approach better, and a smaller Japanese city might find that the Paris approach works better for it.
Cities anywhere might even find that the German approach of not having any express trains works best. This means that planners should consider all stopping patterns, and not just default to what is familiar from nearby cities.