New York City's public transit system is the largest in the world. The ridership in NYC is greater than all 9 of the remaining top 10 US cities combined! NYC is a big and dense city. However, with Los Angeles and Chicago at #2 and #3, this discrepancy is not purely due to city size. I'm going to go out on a limb and propose that the biggest single factor in usage is the number of transit stations which feed systems that are not subject to traffic patterns. How do other worldwide cities rate?
In the major cities of the world, including Tokyo, London, and Paris, subways have long been the most heavily utilized form of public transit, followed by elevated trains. Installing these systems is an astronomically expensive proposition which deters most new growing cities.
While elevated trains are a succesfull part of any transit system, they have gained a bad reputation, primarily due to the unsightlyness and noise of double-rail steel-girder "L-trains" in major cities such as Chicago and New York (photo). These platforms are extremely wide to make room for bi-directional double-rail trains, and their steel-girders rust and rattle, making them a nusance in the neighborhood.
Modern elevated trains, such as in Tokyo and San Francisco (photo), are built with cement platforms. These trains are double-rail systems which take up considerable space and have considerable installation cost. However, they address some of the unsighlyness because they do not rust and the platforms remain stable and relatively quiet for years. San Francisco's BART is criticized for using custom gague tracks which require custom cars and cause skyrocketing costs. (another opinion).
Proponents hail the monorail as the answer. By using a single central rail, the platform size requirements are significantly reduced. However, my experience is that true monorail designs suffer in rideability and speed because of the awkwardness of clinging to a central rail.
London's subways system is certainly one of the most extensive, involving over 275 stations and 253 miles of underground tunnels, in some cases stacked three levels deep. However, skyrocketing costs seem to have affected the Tube as well. The latest work, known as the Jubilee Line Extension, is the most expensive railway ever built, at a cost of US $330M per kilometre. Unlike San Francisco's BART, however, this cost was largly sunk into the civil structures surrounding the trains and rails, not the system itself.
Japan's train system is impressive, totaling more than 22,000 miles of rail routes across japan, with over 150 miles of subway routes in Tokyo, it is the fourth largest in the world after New York, London, and Paris. However, more impressive may be the 200 miles of subway trains in 8 of their other major cities, including Osaka, Nagoya, and Kyoto.
All of these cities pail in comparison to New York's amazing MTA Network, which includes more than 2000 miles of track, including over 700 miles of subway.
However, size isn't everything. Moscow's metro is by far the most trafficed system in the world, with almost 9 million rides per day, despite having under 150 miles of track.
These major cities seem to have gained a critical mass in public-transit which has vaulted them far above and ahead of other cities. Few cities not mentioned here are making any serious motions towards the high percentage of ridership per capita that these cities have reached. The relative economy of road sharing trams have caused cities to compromise, installing hundreds of miles of public transit which gets stuck in traffic. Ridership on these systems is far lower, as they don't solve problems for existing car commuters. We need more organized efforts towards sharing information about and lowering the cost of installing non-traffic dependent public transit systems. What technology and community advances can improve the penetration of quality public transit in our cities?
The urbanaut is one creative design compromise. It uses a central-rail for guidence, while resting on a slightly wider flat track for stability. This system results in a platform which is half of the car width, and in theory, comparably inexpensive to install. However, designs often look good on paper, and no current installations exist.