(CNN) From the single, circular line of Glasgow to the cobweb-like network that is London, underground train systems have been a cheap, reliable and convenient mode of transport for decades.

But what about the others?

But what other systems rank among "the best?"

Hong Kong's MTR (mass transit railway) is renowned for being clean and fast.

There's free Wi-Fi in every station, facilities such as tactile flooring and Braille plates for travelers with disabilities and public washrooms, shops, banks and takeaway food outlets inside many stations or close to their exits.

There's no timetable for commuters -- trains just turn up every few minutes, sooner during peak periods -- and it's dead simple for visitors to buy a ticket via the automated machines.

It has possibly the world's most convenient Airport Express service, with departures every 10 minutes or so.

The website offers handy one-day itineraries for Hong Kong tourists keen to shop, eat or discover the local culture

Occasionally the walk from one concourse to the next can be lengthy and some stations get ridiculously crowded during peak hours (looking at you TST, Causeway Bay).

But that just gives more time to watch and wonder as this 211-kilometer, 150-station system copes easily with its 3.4 million passengers every day.

Seoul's metro system has TV, heated seats and cell phone service.

In addition to being one of the only metro systems in the world with cell phone service and Wi-Fi, many of the Seoul subway trains are outfitted with TVs and are climate controlled. We love the toasty, heated seats in the winter.

"Many subway planners come to Korea and are really blown away by the technology that we have in place," says Jung-whan Kim from Seoul Metro's media team. "It's a big showpiece for Korea's emphasis on IT."

The only downside is the early closing time -- around midnight on weekdays, a little earlier on weekends -- considering how obsessed the city is with nightlife.

The Singapore Mass Rapid transit (SMRT) is probably the fastest way to explore the city.

Some use it to seek refuge from the heat outside, lapping up the air-conditioned comfort.

The system gets demerits for lack of EZ-Link ticket card machines at some stations, meaning frequent lengthy lines for travelers needing to top up or buy a ticket.

The London Underground was the world's first underground railway, opened in 1863.

But for history, for great underground busking and for something relatively cheap in an expensive city, it's hard to beat.

Despite all the grumbling, the Underground ferries more than 1 billion journeys per year. Not bad for something that old.

Roughly 4.5 million people travel on the Paris metro daily.

With more than 1.5 billion passengers a year, Paris Metro is in the top-five for busiest city-rail services in the world.

The Paris Metro does lose some points for not having automatically opening doors. This hints at the average age of the carriages and suggests a need to spend a little on upgrades.

A giant mural of Spanish flamenco artist Paco de Lucia can be found at Madrid's Paco de Lucia station on the Line 9 of the metro.

At 294 kilometers, Madrid has the sixth-longest metro system in the world. But on top of that is another 386 kilometers of suburban rail services.

All up, Madrid's railway serves 1.5 billion passengers each year with 21 lines and 396 stations. Impressive, particularly given that Madrid's population is only 6.5 million.

Madrid Metro has 1,656 escalators, the most of any system in the world.

On New York's subway, you can actually get arrested for putting your feet on a seat.

It would be grossly unjust to leave out the city whose subway system, at least, never sleeps.

New York City's MTA subway lines are doubled up so all local and express trains can run simultaneously along the same routes, 24 hours a day.

And even when carrying out major work on a line, only a single track is decommissioned, leaving a reduced but still-open service. That's planning.

Speaking of musicians, a trip on the MTA isn't complete without encountering a busker (or beggar) doing the rounds of carriages.

Here, a Shinkansen bullet train waits at Tokyo Train Station. The Shinkansen is a network of high-speed railway lines in Japan.

Some 102 train lines, an estimated 14 billion passengers per year. By most measures, Tokyo should take first place on anyone's list of best metros.

Successfully navigating Tokyo by train (and working out which station exit to use) is a proud moment for any traveler.

For some a positive, others a negative, you risk social-pariah status if you ever talk on your phone while moving. That's consideration for others at its best. Or worst.

Guangzhou didn't have a metro until 1997, now it also has a high-speed train to Beijing, pictured here.

Infrastructure investment exploded in 2004 when the city was awarded the 2010 Asian Games. In the ensuing six years, the council spent $11 billion (RMB 70 billion) on the metro system.

For going from absolute zero in 1992 to eight lines, 144 stations, 236 kilometers of track and 1.2 billion passengers in 2008, and for the 48-minute express-trip to Hong Kong (which opened in 2015), Guangzhou must get a mention.

Editor's note: This article was previously published in 2013. It was reformatted and republished in 2017.

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What are the world's best metro systems? - CNN