Dynamically changing express subway service

Four-track subways, like the ones in New York, allow both express and local service to run side-by-side, increasing capacity and speed, but the locations of express stops are fixed. Stations which are not served by express trains simply have no platforms adjacent to the express tracks, so express trains cannot stop at them. It is difficult or impossible to refrofit express platforms to an existing station where express service is now justified, e.g. because of new development. An example of this is 14th Street-6th Avenue station on the New York Subway - on the 6th Avenue Line, it has no express platforms, but all other north-south stations on 14th Street in Manhattan are served by express service. Express service is never run on the local tracks, since it is dangerous for trains to run at high speed beside an occupied platform. Another problem occurs at night, when only local service is usually run - during track maintenance on the local tracks, it is sometimes necessary to run trains non-stop on the express tracks.

The solution to this problem is to construct a central platform between the local and the express tracks at every station, and to install platform screen doors throughout. These doors only open when a train is stopped at a station, and are closed at all other times. They protect passengers from moving trains, whether they are stopping at a station or not, prevent passengers from touching the third rail, and allow subway stations to be heated and air conditioned. As a result, express trains can dynamically change their stopping patterns based on time of day, and new express service can be added at stations which previously only had local service. Furthermore, either track in one direction can be used for local service while track maintenance is performed on the other track (at night, on weekends, etc.) allowing 24-hour service.