This week we've been trying to work out which city has the largest metro in the world. It was surprisingly complicated.

So, for your delectation, here's the whole, horribly over complicated debate.

There are more than 150 metro systems on the planet. Exactly how many, though, is surprisingly difficult to pin down: there's some debate about which systems count, or whether to count integrated networks run by multiple operators as one metro or several.

Working out which metro is the largest is a similarly difficult exercise. Does largest mean “longest”? Most stations? Biggest ridership?

There probably isn't a definitive answer: too much depends on how you understand the question. But what the hell, we've started this now, so this week we're going to look at each possibility in turn. 

Which metro has the most stations?

That looks straightforward enough, doesn’t it? Can’t possibly be any complicating factors here. Right?

Stations are a pretty important consideration when dealing with metros: after all, without places to get on or off, there's not much point in having a train.

And, in this category at least there seems to be a clear winner: the New York Subway has a record breaking 468 stations, roughly three-fifths of which are underground. So, the answer is New York. Here they all are:

A poster listing all New York's subway stations. Image: Alex Daly & Hamish Smyth.

That was easy.

Except, well, looking at things more closely – this got complicated surprisingly quickly – it might not have 468 at all. By "international standards", apparently, it NYC only has 421 subway stations.

So how is there so much uncertainty about whether 47 New York subway stations actually exist? The main reason seems to be that the Metropolitan Transit Authority counts some “station complexes”, such as 14^th Street-Union Square, as two or more stations; most networks would count them as one. You'd think it'd be easy to work out how much stations a metro network has, but no, apparently not.

Anyway, what we can say for certain is that the subway has 368km of routes and currently operates 24 services which, we think, is the highest number in the world. It's a pretty big network, is what we're saying here. And that's without counting things like PATH.

Despite the chronic uncertainty over exactly how many New York subway stations there are, it's pretty clear that there are more than on any other network. No other system comes close: the Shanghai Metro is in distant second with its 12 lines and 337 stations, many of which come with fancy features like sliding safety doors.

Platform screen doors installed at Shanghai's Xujiahui Station. Image: Jianshuo/Wikimedia Commons.

Beijing isn't too far behind, with 319 stations, although this seems to run into similar problems as the figure for New York, and a more accurate count may be 268.

Anyway, here’s the whole Top 10, using the figures as given by the networks themselves:

• NYC Subway – 468
• Shangai Metro – 337
• Bejing Subway – 319
• Seoul* Subway – 311
• Paris Metro – 303
• Madrid Metro – 301
• London Underground – 270
• Moscow Metro – 196
• Mexico City Metro – 195
• Tokyo Metro – 179

You notice that asterisk next to Seoul? That’s because we’re only counting lines 1-9, and not a whole bunch of other stuff that may or may not be part of the network.

Confused? Just you wait until we try to answer the next one:

Which metro has the most stations?

You'd think, by restricting ourselves to a simple, one-dimensional numerical measure, that working out which city had the longest metro system in the world would be simple, wouldn't you?

Ha. No such luck. This time (spoilers), the root of the confusion lies in the vexed question of what counts as one metro network.

One oft-cited candidate for the world’s longest metro network is the one in Seoul, which only opened in 1974 but by 2013 included 987.5km of route on 18 lines. That's pretty much enough to get you from London to Marseille. No other city comes close.

So why is this question remotely contentious? Because it's surprisingly unclear whether that metro should be counted as one system or several. It includes lines 1-9, the subway proper, which is operated by the Seoul Metropolitan Rapid Transit Corporation and the Seoul Metro (with which it'll merge next year).

But it also includes routes run by national rail operator Korail. Most contentiously, it includes lines run by satellite cities, such as the Incheon Transit Corporation, which operates trains in, well, Incheon.

An extract from a map of Seoul's complete metro network created by Wikipedia User IRTC1015. You can see the terrifyingly complicated full version here.

All these operators provide a single integrated network – but it's still the equvialent of measuring the London Underground by counting Overground, DLR, and so forth, or of counting the RER and Transilien as part of the Paris Metro. We are in danger, in other words, of comparing apples and oranges.

So what if you try to stick to apples alone? Look at lines 1-9 alone, and the network measures only 311km.

Which is quite a lot smaller and probably not the biggest in the world at all.

Other candidates for the top slot can’t promise the 900km+ of route that the wider definitions of the Seoul metro do, but the figures they do cite are probably less contentious.

One is the Shanghai Metro, which runs to 548km and counting. In 2013, it achieved the distinction of becoming the first Chinese metro network to cross provincial boundaries, when line 11 was extended into the satellite city of Kunshan in neighbouring Jiangsu province.

There's talk of extending it further, to connect up with metro systems of the cities of Suzhou and Wuxi, both of which are 100km away or more, too. Not bad given that Shanghai didn't open its first metro line until 1993.

It isn't the only monster subway system that's sprouted in China recently either. The Beijing Subway, first opened in 1969, is the country's oldest, so it got a bit of a head start on Shanghai. Today, it runs 18 lines, serves 319 stations, and stretches for 527km.

Schematic map of Beijing's subway, created by Wikipedia Users Ran and Hat600.

This is another one of those networks which has more than one operator: this one's split between the state-owned Beijing Mass Transit Railway Operation Corp (15 lines) and the Beijing MTR Corp (a joint venture with the Hong Kong transit authorities, which operates three). Between them, in 2014, they carried 3.4bn passengers.

One interesting feature of Beijing's metro is its station names, which, translated literally, mean things like Smooth Justice, Heavenly Peace Gate and (less appealingly) Cholera Camp. So, there you go.

Here, best we can tell, with all the caveats about different cities playing by different rules here, is the top 10 of metros proper:

• Shanghai Metro – 548km
• Beijing Subway – 527km
• London Underground – 402km
• New York Subway – 373km
• Seoul Subway – 332km
• Moscow Metro – 328km
• Madrid Metro – 294km
• Guangzhou Metro – 240km
• Mexico City Metro – 227km
• Nanjing Metro – 224km

Anyway. Next we're going to try counting people. That's where things get really complicated.

Which city has the busiest metro system?

There’s something inherently about awkward about phrases like “1.5bn people a year ride on the Paris Metro”. It doesn’t mean that a fifth of the world are hanging around Châtelet–Les Halles station at least once a year, obviously, it just means that there are that many journeys undertaken.

Anyway. Until very recently, on the question of which metro system carried the most passengers – had the highest ridership, in the jargon – there was a clear winner. The Tokyo Underground Railway Company launched Japan's first underground railway, the Ginza line between Ueno and Asakusa, in 1927. It was just 2.2 km long, but nonetheless, the line became so popular that passengers would queue up, sometimes waiting for over two hours just to ride the metro for five minutes.

Nearly nine decades later, the privately run Tokyo Metro runs nine lines, while the publically-owned Toei Subway operates another four and the Tokyo Waterfront Area Rapid Transit another. Between them they cover 290 stations – and carry a colossal 3.3bn passengers a year, or over 8m a day.

Unsurprisingly, the network has become a byword for overcrowding – a sort of metaphor for Japan's work culture. The concept of “pushers”, in which guards help passengers by shoving them into crowded subway trains, seems to have started in New York. But these days, the “osiyas” (literally, Japanese for “person who pushes for a living”) are associated mostly with Toyko's crowded metro.

Pushers at work. Screenshot from The Fat Finger on YouTube.

At some point in the last couple of years, however, Tokyo may have lost its crown as the world's most crowded. Beijing’s has 18 lines, run by two operators: between them they carried an estimated 3.4bn passengers in 2014.

We say “may” because, as ever, it is difficult to be sure we're comparing like-with-like here: a journey involving two operators and three different lines may be counted once, twice, or three times, depending on the statistical rules applied by the local authorities. At present, then, it's difficult to be sure that Beijing has overtaken Tokyo. If it hasn't, though, it seems almost certain that, in the not too distant future, it will.

Other networks are racing up behind, too. The Shanghai metro only opened in 1993, but in just over 20 years it's expanded to include 327 stations on 14 lines. By 2014 it was already carrying 2.8bn passengers a year. At the end of that year it's believed to have achieved a world record, when it carried 10.3m passengers in a single day.

Not far behind that is our own friend the Seoul Subway, where lines 1-9 carry 2.6bn passengers per year. (The extended network that we talked about last time carries considerably more.)

• Beijing Subway – 3.4bn
• Tokyo Subway* – 3.2bn
• Shanghai Metro – 2.8bn
• Seoul Subway** – 2.6bn
• Moscow Metro – 2.5bn
• Guangzhou Metro – 2.3bn
• New York City Subway – 1.8bn
• Hong Kong MTR – 1.7bn
• Mexico City Metro – 1.6bn
• Paris Métro – 1.5bn

*Includes the Tokyo Metro, the Toei Subway, and the Rinkai Line.

**Lines 1-9 only

The London Underground is bubbling under in 11th place with just 1.3bn. And you thought the Central Line got crowded of a morning.

So which metro should we call the world's biggest? Is it Toyko for being the busiest? Seoul for its length? New York for its station numbers? Or Shanghai for placing well in all categories?

The answer, alas, has to be “it depends how you count”. Sorry. We did try to warn you.

Research: Suren Prasad.