What are the world's best metro systems?
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.
The London Underground, also known as "The Tube," celebrates its 150th anniversary in 2013. Having stood the test of time it ranks as one of the greatest metro systems in the world. (Share with us your most memorable Tube journey.)
But what about the others?
Copenhagen Metro has a driver-less rail system that runs 24 hours a day.
São Paulo Metro has a comparatively small 74 kilometers of track, yet carries 3.3 million passengers each day.
Montreal has one of the lowest carbon footprints for metro networks in the world.
Many CNN readers nominated Moscow in the comments section below.
But what other systems would they rank among "the best?"
We've taken onboard readers' comments and suggestions to update this article.
Here are a few underground rail systems we think deserve credit -- let us know your thoughts in the comments below.
Praise for Hong Kong’s MTR gushes from every traveler who’s ever set a toe inside the immaculately clean, well-signposted, cheap, regular, convenient system that connects most corners of the city, from the crowded bowels of Wan Chai to the rural(ish) villages of Tai Po.
“…one of the most efficient people carriers in the world,” says reviewer swissfondue on VirtualTourist.
There’s free Wi-Fi in 42 stations, 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.
And then there's the Octopus card -- possibly the world's greatest transport payment system, which can also be used in convenience stores, restaurants and other places. Cities like Melbourne should scream with jealousy.
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.
Also on CNNGo: 10 epic train journeys
Operated by three different companies (two of them state-funded), the Seoul metro system carries almost 7 million passengers per day on nine lines.
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.
After an amalgamation of several transit-service-providers in 2000, SMRT has grown to 600 million passengers per year.
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 Tube was the world's first underground metro, opening in 1863 … and they've not done terribly much since. Only in the current, two-decade-long upgrade plan have air-conditioned carriages been introduced.
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.
Also on CNN: 10 tips for London Tube survival
The City of Light's metro is unusually dense, with 245 stations on 14 lines, in just 87 square kilometers of the city. Parisians, apparently, don't like to walk.
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.
Also on CNN: Europe's most beautiful metro stations
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.
The underground stations are so huge that they can hold public events, such as the three-day fitness festival in May 2011, which attracted 2,600 visitors. One station contains a 200-square-meter archaeological museum.
Madrid Metro has 1,656 escalators, the most of any system in the world.
New York City
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.
Hurricane Sandy hit the network hard in 2012, though it was quick to rebound with only a few areas yet to return to 100 percent.
Extra points must go to New York City for the MTA's Music Under New York program, supporting local musicians.
Speaking of musicians, a trip on the MTA isn't complete without encountering a busker (or beggar) doing the rounds of carriages.
Tokyo's rail system is legendary. Super-fast, super-punctual, super-everything.
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.
After failing five times in 30 years to create a metro system, Guangzhou's first metro line was finally opened in 1997 and a second line was opened in 2002.
Infrastructure investment exploded in 2004 when the city was awarded the 2010 Asian Games. In the ensuing six years, the council spent RMB 70 billion (US$11 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 upcoming 48-minute express-trip to Hong Kong (opening in 2015), Guangzhou must get a mention.
Got your own favorite? Have a great metro story? Tell us in the comments.