[fennecfoxen]

The core problem is that the New York subway system isn't one subway system, it's three:

Interboro Rapid Transit (IRT), which built a lot of elevated lines from the Bronx into Manhattan (all the Manhattan lines have been torn down, including the second and third avenue el, which were never replaced with the promised Second Avenue Subway)... they ran shorter, narrower train cars and their remaining lines turned into the 1234567 and Times Square Shuttle (internally, the 0)

Brooklyn-Manhattan transit (BMT), which ran a bunch of trains into Brooklyn...

and the Independent Subway System (IND), which was run by the city with BMT car standards and was responsible for the Manhattan portions of the ABCDEF trains, the G, and the Rockaway Shuttle (internally the H) and bought a few more rail lines in Brooklyn as well (the tail ends of the A and the F).

If you look up the maps, the places where the subway really looks awkwardly constructed, the stations are sprawling, the tracks are at funny angles... it's because they were competitors. Why does the 2/3 cross the L out in Brooklyn without a transfer when the stations are right on top of each other? They were run by competitors. Why do the F/A/C/G shun the other lines as they plow through downtown Brooklyn? Why do you need to walk from the 6 to the F at 63rd Street? Why can't you transfer between the Hewes Street and Broadway (G) stations if they're right on top of each other? Why can't anyone figure out how to navigate Court Street? Why, for years, was there a transfer between the BDFM and downtown 6 at Bleeker St, but not the uptown 6? (That silly little bubble in the subway map.)

To this day there is exactly one spot where an IRT train shares a platform with a BMT train (Queensboro Plaza). And there are essentially two places where trains transfer between the old IND and BMT tracks (at the Manhattan Bridge, and Queens Plaza -- the third, under Central Park between the Q and the F, occasionally figures in maintenance-related detours).

The good news is that New York is lousy with subway stations, and if there's a disruption on one line, there's a very good chance you can still get where you're going on another. The bad news is that's fundamentally confusing.

[beejiu]

You can argue much the same about London underground -- it too is a mishmash of previously competing railway lines, and has grown in complexity to include all manner of railways -- some cut-and-cover, some deep-tunnel, some overground. For the uninitiated, it is a very complex system. The problem with our system is that it has to inherit decisions made over 150 years ago in the steam age.

[fennecfoxen]

The London Underground does have a few advantages over New York, though -- they were able to build deep tube lines with ease because of the geology (the rock under Manhattan being much tougher to tunnel through). New York subways are almost entirely cut-and-cover, which places substantial constraints on where they run (under city streets, where you have to worry about nearby basements, and water mains and the like) and how much space there is to connect them to each other. This is also a reason they have more sharp curves that make terrible metal-on-metal squeaking sounds.

London's tube stations have a lot of space between their platforms, enough to provide a network of hallways with separate entrance and exit passageways and escalators (some of them are equipped with switchable wayfinding signs so you can have different ways out at morning and evening rush hour). New York doesn't have anything like that anywhere; you'll often have a single set of stairs from the street down to one side of the tracks (with no provision for transferring to the other side at all, to say nothing of elevators). If it's a nice, modern station the tracks will be a little deeper so the station entrance serves as an overpass over the tracks, but other times the transfers will only be available by walking along another subway platform.

[saryant]

Tokyo has two competing subway systems as well, the private Toei and the municipality Tokyo Metro. Fortunately those are so well integrated the average visitor would never even know.