The New York City subway, opened in 1904, is one of the few public transit systems worldwide that runs 24 hours a day, seven days a week. It is also one of the most well-connected underground networks, with 24 lines, 660 miles of operational track and 468 subway stations – more than any other metro system in the world. For this reason, around 4.5 million people ride the subway every day.

However, some argue that the New York subway could be safer. Last year, 55 people died on the tracks. This year, following a string of deaths early in the year, politicians and citizens alike are imploring the Metropolitan Transportation Authority (MTA) to follow the lead of cities in Asia and Europe that have installed barriers along the platform edges to prevent suicides and accidents. But so far, the MTA has said it is too expensive to install such barriers in stations that are more than 100 years old.

In Hong Kong, a 2008 study by the University of Hong Kong found that installing platform safety doors resulted in a 60% reduction in railway suicides. As such, the Mass Transit Railway (MTR) added platform safety doors to all of its 84 stations, finishing the multi-year construction project in 2011. Indeed, the MTR’s underground had zero fatalities in 2011, the most recent year for public data. The Hong Kong metro, which opened in 1975, receives high praise from riders for efficiency as well as safety. With 175km of track over 10 lines, the MTR accommodates 4.15 million passengers every day.

The Copenhagen Metro, which became operational in 2002, combines the best of New York and Hong Kong. Unlike other European public transit systems, it stays open 24 hours a day, seven days a week. Plus it focuses on safety with the use of both platform barriers and driverless trains, which many believe enhance safety and reliability by removing human error from the equation.

Customer satisfaction with the subway landed Denmark’s capital the MetroRail prize for the World’s Best Metro in 2010. The Copenhagen Metro does not, however, offer the size of the MTA or MTR, operating only two lines over 21km of track with 22 stations. Still, for Copenhagen’s size, the ridership is impressive – as of 2010, annual ridership was around 260,000. A new line is expected to launch by 2018, adding 17 more stations.

Another award-winning metro can be found in Singapore, where the Singapore Mass Rapid Transit (SMRT) has been named the most energy-efficient metro in the world – with such innovations as escalators that stop moving when not in use and an inversion technology that recovers and recycles excess energy from braking trains. Established in 1987, the Singapore metro runs trains on four lines servicing 131 stations. Its tracks, which cover 175.3km total, are ridden by about 2.4 million people each day.

The oldest metro in the world, though, is the London Underground, which opened in 1863. The Tube is in the middle of celebrating its 150th anniversary this year, with history-focused events and activities taking place at Covent Garden’s London Transport Museum throughout 2013. With 250 miles of track, 11 subway lines, 270 stations and around three million passengers each day, the Tube is one of the largest underground railway systems in the world.  

Meanwhile, one of the world’s most impressive subway systems is the Tokyo Metro, which caters to an incredible 8.6 million passengers each day while maintaining a far-reaching reputation for efficiency. The metro began running in 1927 and has since grown to encompass nine lines, 168 stations and 320km of track. When it comes to comfort, though, Tokyo’s crowded trains are surpassed by the Seoul Metropolitan Subway, which has been around since 1974. Carrying nearly seven million passengers each day on nine lines and 327km of track, the Seoul subway has climate-controlled train cars with mobile phone reception, wireless internet and televisions. In the winter, riders can even look forward to heated seats.

You won’t find much need for heated seats in Brazil, where the São Paulo Metro has become one of the most ridden subway systems in South America since its launch in 1974. Its five lines span 74.2km of track and carry around 4.4 million passengers each day to 64 stations. It is Chile however, that has the continent’s most extensive underground system, with the Santiago Metro covering 108 stations and 103km of railway on five lines. However, the Santiago Metro, which opened in 1975, carries fewer passengers than its Brazilian counterpart, with 2.3 million people riding daily.

It may be difficult to believe, but in all of Africa, only two cities have subway systems: Algiers, which just launched its metro at the end of 2011, and Cairo, where the subway has been around since 1987. Nearly four million people ride the Cairo Metro each day, a network stretching out over 69.8km of track and servicing 57 stations. The subway only has two lines, though, and is currently working on building a third. The Cairo Metro is much larger than the Algiers Metro, which consists of one line, carrying around 300,000 riders each day to just 10 stations via 9.2km of track.

Travelwise is a BBC Travel column that goes behind the travel stories to answer common questions, satisfy uncommon curiosities and uncover some of the mystery surrounding travel. If you have a burning travel question, contact Travelwise.