Two-track subways have inherent maintenance problems. New York is mostly four-track, so sections of track can be shut down for total replacement. They also have a huge number of track switches and interlockings, and can reroute onto alternate tracks easily. NYC can also run trains 24/7. Two-track systems can't do that. One of the legacies of the Tammany Hall regime is that the infrastructure from about 1900 was way overbuilt. But the real reason NYC is four-track is that the subways replaced elevated lines that were at capacity with two tracks.

Few other subways are four-track. The Broad Street Line in Philadelphia is four-track, but is underutilized. A few sections of the Chicago MTA are four track. The London Underground has the two-track problem. (Worse, that system is older and has undersized tunnels. They have to build special maintenance machines that will fit in their tunnels.)

In the US, there are probably two systems that could justify four-track - SF's BART and Washington's Metro.[1] But it took both a long time for each to build the traffic to justify it. BART has a lot of elevated sections, and four-track elevated lines roof over an entire street. (NYC used to have some; getting rid of them was one of the reasons the subway system was built.)

In practice, two-track systems do heavy maintenance during weekends and holidays. This takes careful planning, special equipment, and lots of competent, hard-working people. But it's done routinely by the big systems. SF Muni is shutting down their underground lines from 9PM to 2AM, and the Twin Peaks tunnel will be closed for 21 weekends starting in June. It's more expensive than a full shutdown, but has less impact on customers.

[1] http://greatergreaterwashington.org/post/3760/was-the-lack-o...