It’s a commonplace in New York that the New York City Subway is almost the only one that runs 24/7, and that the rest – PATH, PATCO, and two lines of the Chicago L – are small operations. The reason for this operating plan is that the main Manhattan trunklines have four tracks, making it feasible to shut down tracks for weekend and late-night maintenance and skip a few stations in one direction. Occasionally, even midday midweek service is disrupted. This leads to complaints from passengers who actually ride transit in the off-peak, as well as various politicians, and exhortations from political defenders of the MTA that it’s a necessary byproduct of 24/7 operation.
In fact, there’s one additional system not mentioned above: the Copenhagen Metro, which began 24/7 operation in 2009. Although around-the-clock operation on weekends is common in some European cities, such as Berlin, Copenhagen took the extra step to run 24/7 reliably. It has only two tracks, like some lines in New York, but made sure it would be possible to single-track at night for maintenance. Late-night headways in Copenhagen are 20 minutes, like in New York, and this gives enough time to reduce long segments to a single track and run wrong-way service. Copenhagen’s trains are automated and this helps with wrong-way signaling, but it’s not a prerequisite and wrong-way operation is already done late at night on the subway in New York.
What this means is that there’s a technical solution to the problem of late-night and weekend service disruptions: make sure that there are crossovers placed at regular intervals to allow 20-minute service on single track. Installing switches requires extra capital construction money, but is orders of magnitude cheaper than building extra tunnels, and would make late-night maintenance much easier. Headways are such that a switch would be required every 7 or 8 minutes, which means every 2.5-5 km. At some places, crossovers already exist at that density, for example at all four tunnels from Queens to Manhattan, and all that’s required is schedule modification.
The result would still not be as satisfactory as in Copenhagen, ironically because of the multi-track trunklines. Under the slow-fast-fast-slow system used in New York, as well as most other four-track lines, it’s impossible for a local train to cross over to the opposite track without fouling the express tracks. This would create serious problems even on the three-track lines in Queens and the Bronx, since extra switching moves would be required, shortening the acceptable crossover spacing. It would still be possible, say with crossovers 6-7 minutes apart, but the maintenance requirements would be higher.
On the four-track mainlines, I don’t see any solution that unequivocally improves on the status quo. It’s possible to have the same crossovers, but at even tighter spacing, and without any express traffic. Weekend express traffic could possibly still be retained, but not late-night express trains, and late-night frequency would be reduced to 20 minutes even on combined lines, for example the local 1/2 in Manhattan.
What this means for future trunklines is that, if four-tracking is required for capacity or for express service, it should not run as was built in New York a hundred years ago. Instead, the slow tracks should be in the middle, and the fast tracks on the outside; this allows more operational flexibility as well as short-turning local trains, at the cost of making it harder to build infill stations. While the subway short-turns some local trains, for example the C at 168th and at Euclid, this requires flying junctions, which contributed to the IND’s excessive cost.
Maximum flexibility could be obtained by building every station with two island platforms, as if it were an express station, and having express trains skip low-traffic stations. This way, two tracks could be shut down for maintenance along the entire line with no ill effect on reliability, except that retaining express service would required timed overtakes. The problem is of course the much higher cost of such a line, especially if it is underground.
For underground lines, there’s very rarely a reason to four-track. Washington may complain about lack of flexibility and express service, but modern subway lines with good rolling stock and wide curves can achieve acceptable average speed even with medium stop spacing. The Copenhagen Metro averages 40 km/h, a speed previously reserved for systems with very long (~1.6 km) interstations such as the Moscow Metro, even though its stop spacing is just 1 km. Capacity is the only serious drawback of two-track lines, but if it is so pressing then the city should built two separate two-track lines, which with tunnel boring machines cost about the same as one four-track line.