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.
Hey, they're mostly General Railway Signal's NX system, the first intelligent user interface ever. It takes a huge number of big relays to express all the logic, but it's a good system. Everything in rail control since NX looks a lot like NX, even if it's on a screen.
Before NX, someone had to manually set up routes through an interlocking, setting all the switches and signals. The interlocking machine would prevent conflicts (the levers would not move when pulled) but the dispatcher had to figure out the routes. With NX (eNtry eXit), you push a button to select the track on which a train is approaching. Lights then light up for all the places it can exit the interlocking. The system takes into account other trains, previously set up routes, and even switches and track locked out of service for maintenance. You push one of the exit lights. The system then sets up all the switches and signals to get the train out of the interlocking at the desired exit. As the train progresses along the path, the resources (track sections and switches) are released behind it, and become available for other trains.
NX works well. Many later systems were worse. BART had big problems with their 1960s advanced electronic system, which had to be replaced.
(That video shows an older lever system. That's the only one on the NYC subway system.)
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...