For commuter rail, even more so than urban transit, there is a tradeoff involving speed, cost, and coverage. Higher speed is useful all else being equal, but all else is frequently not equal. American commuter rail is on average faster than European and East Asian commuter rail, but fails because relative to the distances people travel and the amount of sprawl it must compete with it is quite slow. Because of the need for higher speed, my previous commuter rail modernization proposals have featured average speeds higher than those that are normally found elsewhere.
That said, speed should be increased by means of better rolling stock, adequate maintenance, and better timetable adherence leading to less schedule padding. The American practice of running low-frequency trains from each suburban station expressing to the main city station makes service worse rather than better. Consider the following practices:
In Paris, RER trains are more or less local. There are some trains skipping stops, but in RATP territory (see schedules here), trains will just skip a few stops, rather than running truly limited-stop. In SNCF territory there are some express trains. In addition, the Transilien trains run on legacy routes even in the city, and run express outside it; often they’ll run nonstop between Paris and the terminus of the parallel RER line.
In Berlin, the S-Bahn lines run local. As in Paris, there are regional lines that are separate from the metro-like S-Bahn, and those make fewer stops in the urban core.
In Tokyo, there are local trains and rapid trains. Local trains make all stops. Rapid trains come in several flavors – they only stop at select stations, but sometimes there are several levels of rapid trains (all on the same tracks). The busiest lines have a track pair dedicated to local trains, and a track pair dedicated to express trains, and then there are usually two consistent express patterns, one more express than the other. The local trains (Yamanote, Keihin-Tohoku, Chuo-Sobu) run on dedicated tracks and are very metro-y in their lack of branching, and the rapid trains run on separate lines and look more legacy, just without the brand separation of France and Germany. In addition, there’s less separation of infrastructure.
For example, the Chuo-Sobu Line has local trains running to Mitaka, and the Chuo Rapid Line has trains making limited stops to Mitaka and local stops farther west as well as trains making limited stops all the way, with timed overtakes at the express stations; the express trains’ stopping pattern is consistent. Consult this schedule for details (click on “interval timetable”).
The speed of all of those lines is quite low – for example, the RER A averages 47 km/h between Boissy-Saint-Léger and Saint-Germain-en-Laye. However, this comes from short station spacing, coming from the fact that they serve dense urban areas, with reasonably dense suburbs. The need for speed in Paris is much less than in Boston or even New York. In Paris, 20-30 kilometers out of the city one already leaves the built-up area. Even in Tokyo, a much larger city, Takao, 53 kilometers from Tokyo Station, is near the end of the urban area. In contrast, Providence is 70 kilometers from Boston, and Ronkonkoma is 80 kilometers from Penn Station.