This idea would only work on a circular metro line. Build four tracks, two for express trains, two for local trains. The local trains operate like any normal metro, stopping at every stop.
The express trains are driverless and programmed to travel at such a speed that travelling between *any* two adjacent stations takes exactly the same time (say 1 minute) no matter how far apart the stations are.
To begin with, you put an express train at every single station on the circular line. Then, all express trains depart at the exactly same time. The trains all reach cruising speed and skip 4 stations. 5 minutes later all trains arrive at exactly the same time, 5 stations further from where they started.
The advantages: It will be possible to take express trains from all stations, not just a small number of hub stations (like it's the case on the New York subway). This will a) reduce overcrowding since every station now acts as a hub b) reduce journey times, since in the worst case a passenger only needs to make one change (express+local) rather than two changes (local+express+local)-- kinemojo, Sep 01 2007random, halfbakery