S-Bahns and similar systems have two defining features. One has been hashed to death on this blog: they reuse legacy rail lines, allowing urban rapid transit to extend arbitrarily deep into suburbia. The other, common also to many other transit technologies, is that they branch extensively, allowing them to run many services on the outer ends, where there’s no demand for rapid transit frequency, while interlining to produce high frequency in the center, where there is.
Since branching is a service planning decision independent of technology, any technology could branch. The branching-friendliest technology is subway-surface: the central subway segment has higher capacity measured in trains per hour than the outer surface segments, and this requires branching. For examples, consider the Boston Green Line, Muni Metro, the Frankfurt U-Bahn, and SEPTA’s Subway-Surface lines. However, even when the entire line is rapid transit, branching is useful to ensure higher service where there is higher demand, and infrastructure improvements will typically focus on boosting capacity in the center. For example, the RER A has moving-block signaling allowing 30 peak-direction trains per hour in the center, but fixed-block signaling on the branches, which do not need such capacity.
Even when rapid transit is built separate from both light rail and mainline rail, branching is useful for lines going into the suburbs or even outer-urban neighborhoods. This is practiced in both New York and London, both of which have extensive branching. Observe further that in both cities, the lines reaching farthest out – the A in the Queens-bound direction and the Metropolitan line in the west – are also the most highly branched.
It’s the opposite situation that is weird. When lines do not branch, there must be a strong outer anchor, or else trains need to run empty outside the center. The alternative is short-turns, and if there’s no space for this, the resulting service patterns can be awkward. Shanghai, which has little branching, runs Line 2 in two segments, a central segment with higher frequency and longer trains and an eastern one with lower frequency and shorter trains; trains do not run through. Beijing has a similar awkwardness with the split between Line 1 and the Batong Line, and Toronto has a split between the Bloor-Danforth line and the technologically incompatible Scarborough rapid transit. (The Sheppard line suffers from the same problem today, but it has the excuse that it was planned to continue west to the Spadina subway rather than stub-ending at Yonge.) Paris has little branching on the Métro as well, but the Metro only serves inner parts of the metro area, many lines have strong outer anchors (for example, La Défense on Line 1), and two others providing some of the farthest-out service branch. The RER branches much more heavily, as befits a suburban system. Tokyo has little branching on the subway proper, but the subway is for the most part inner-urban, and lines continue to the suburbs along commuter lines, which do branch.
In North America, this configuration has been common across a variety of new-build systems, especially ones that should have been S-Bahns. BART does this the most extensively, but the Washington Metro is also highly branched for its size, MARTA branches, the light rail systems branch once more than one line is built, and so on. BART in particular imitated the service planning aspect of commuter rail perfectly, and is an S-Bahn in all but the cost of extending the system further.
The problem with any branching is that it reduces frequency on the branches, potentially scaring away ridership. When a single rapid transit line splits in two it’s rarely a problem, and when city-center service splits into suburban services even more is easy to justify. I think the main issue in urban or inner-suburban cases is that with typical rapid transit frequencies (3-minute peak service or slightly better, a peak-to-base ratio of 2:1 or somewhat less) the trunk has about 5-minute off-peak service, and if it splits into two branches, this means 10-minute service on the branches. If the branching occurs early enough that dense neighborhoods with short-distance travel demand are on branches, it may be too little. In addition, if one branch has much more demand than the other, then it’s usually hard to match frequency on each branch to demand, since it requires trains to be unevenly spaced.
The issue is that branch frequency, 10-15 minutes, is in the transition zone between urban show-up-and-go frequency, where schedules do not matter, and suburban frequency, where they do. It’s perhaps less relevant in small cities with small enough transit systems that even 10-minute service is considered very good, but in large cities, people expect more, creating somewhat of an inner-urban metro envy effect.
That said, 10-minute suburban and outer-urban service can be done clockface, making the average wait much smaller. It is done on the RER A in the midday off-peak, with three 10-minute branches, and could be done with two 10-minute branches quite easily. Likewise, it could be done for 15-minute branches (the RER B already does this); the two A branches in New York have close to 15-minute frequency each, and if New York City Transit’s service planning considered it as a factor instead of focusing more on headway management it could ensure predictable schedules at Ozone Park and the Rockaways.