For many Americans, the word "subway" describes a quick-and-cheap sandwich shop, rather than a mode of transport. This makes sense, of course, given that only a handful of American cities have underground rail networks. On the other hand, subway systems are extremely important to residents of large cities outside the United States. Sometimes, as in the case of the busy subway systems on this list, to the extent that millions of people use them each day.
Beijing Subway, China
While traditional wisdom (and, up until a few years ago, raw numbers) suggest that Tokyo's subway system is the busiest in the world, that title goes to Beijing, whose underground system registered nearly 10 million riders per day last year, by a long shot. Beijing's busy subway is interesting for a couple of reasons, first because of the awful traffic that exists in spite of it and secondly, because before the city was awarded the 2008 Olympics, ridership was dramatically lower. As of 2017, the Beijing Subway system has 19 lines and 345 stations.
Shanghai Metro, China
Tokyo's not currently #2 on the list of world's busiest subway systems, either. No, it's Shanghai: More than 9 million people per day rode the Shanghai Metro in 2016, only slightly fewer than did so in Beijing. Like the Beijing Subway, the Shanghai Metro's most dramatic growth spurt occurred in advance of a high profile international event (The 2010 World Expo). Interestingly, the two system have an almost identical number of stations (Shanghai has 19 more than Beijing as of 2017), although the Shanghai Metro has five fewer lines than the Beijing Subway.
Tokyo Subway, Japan
For decades, the 8.7 million people who ride in the Tokyo Subway system each day were enough to make it the world's busiest metro, but times (and China) have changed. It's important to note, however, that this figure does not include the dozens of private and public railway lines Tokyo's 30+ million residents use to commute every day. It's also interesting to note that while 6.4 million passengers ride the Tokyo Metro's nine lines each day, the other 2.3 million ride exclusively on the four-line Toei Metro, which uses a separate ticketing system entirely. One way to make navigating Tokyo's complicated subway easier is to purchase PASMO or SUICA stored value cards, which work across all systems.
Guangzhou Metro, China
So far in 2017, about 7.36 million people have ridden the Guangzhou Metro each day, which puts it on track to exceed the 2.5 billion riders who used it in 2016. The fact that Guangzhou's 10-line, 186-station metro is world's fourth-busiest is remarkable for a number of reasons, not least of which is how poorly-known the (mega)city is outside of China. It also means that China is home to three of the world's five busiest subway systems, which is pretty crazy when you consider that only one (Beijing) would've cracked the Top 10 just a decade ago.
Seoul Metro, South Korea
The Seoul Metro boasts 7.2 million daily riders as of 2014, which is only shabby when you compare it to the previous four entries on this list of the world's busiest subway systems. 283 stations dot the system's nine lines, which are co-operated by the Seoul Metropolitan Subway Corporation and the national Korail company. An interesting fact about the Seoul Metro is that many of its stations are extremely far underground. Sometimes, you have to go down three or four sets of escalators to reach your train!
Moscow Metro, Russia
The Moscow Metro posted an average daily ridership of 6.7 million in 2014, the latest year for which figures are available. This is remarkable on its own, but what's really beautiful are the Moscow Metro stations themselves—certainly, they stick out on this list. In fact, you could occupy several days riding between the 206 stations of this system, particularly if you visit Moscow during the frigid winter months.
New York City Subway, USA
The fact that New York City's Subway ridership was about 5.7 million riders per day in 2016 says a few things, first among them that America's busiest subway system barely cracks the Top 10 of the world. It also speaks, of course, to a dysfunction within the 468-station system (notably, the world's largest by that measure) of which locals are all too aware. For example, did you know that the recently-opened Second Avenue Subway line took almost a century to complete? The New York City Subway might be the busiest and best-known in America, but it certainly doesn't have anywhere near the best reputation on this list.
Hong Kong MTR, China
Hong Kong is one of the most densely populated places on the planet. You need to consider this fact when you think about why it's home to the eighth-busiest subway system in the world, in spite of how small the map of its rail system looks. In fact, about 4.694 million people ride the Hong Kong Kong MTR each day, in spite of the fact that it has just 12 lines and 68 stations. This will be no surprise to you, of course, if you've ever ridden the Hong Kong MTR yourself—it's crowded even outside of rush hour. This is particularly the case at station in Kowloon, which is Hong Kong's densest area.
Mexico City Metro, Mexico
Spanish might be the world's second-most spoken language, but only one city in the Spanish-speaking world has a subway system that ranks among the world's busiest. In Mexico's capital Mexico City, about 4.4 million riders per day use the Metro system, which has 195 stations and 12 lines. The system is notable for having survived the massive earthquake that shook Mexico City in 1985. It's also notable for being one of only two entries in the Americans on this list of the world's busiest subway systems.
Paris Métro, France
The Paris Métro is Europe's second-busiest subway system, after Moscow—and, notably, ahead of the more ubiquitous London Under. In 2014, the Paris Métro reported an average daily ridership of 4.16 million, about 40% more than London during the same amount of time. Although Paris' Métro isn't the busiest in the world it is one of the densest: All of its 245 stations are built within 34 square miles in the city of Paris. Paris' Metro is dense, and it's extensive, which means you can ride it to almost all the city's important tourist attractions, including the Eiffel Tower, Notre Dame Cathedral and the Louvre Museum, to name just a few.