The answer to this question is harder to find than you think. In most countries, international train traffic is an insignificantly tiny part of total rail traffic, and even where it is important it’s often mostly freight. No-one really cares about international passenger travel, and when they do publish data it can be really bad quality.

But CityMetric cares about international train travel, damn it, and after hours of poring over terribly formatted spreadsheets and reports, we’ve calculated what are (probably) the most popular international trains in the world.

The EU collects a lot of data about international rail travel, making it a good place for us to start. Unfortunately the data comes from each individual member state, and they sometimes contradict each other (Belgium and the UK disagree by nearly a million about how many people travelled between them in 2011) or just don’t bother filling it in (France haven’t updated their stats since about 2009).

Still, we can fill those holes in with old data and by assuming the same number of people cross the border both ways, and we can find the most used international journeys in the EU:

Click to expand.

The most popular by a long way appears to be the UK-France connection through the Channel Tunnel, which sees over 16 million passengers a year – not even counting the nearly 3 million more going between the UK and Belgium.

But… Only about half of that is Eurostar. It looks like the official data counts also counts people using Le Shuttle – the trains that run and back forth carrying motor vehicles under the Channel. If you’re sitting inside a car that’s inside a train, are you a train passenger or just a motorist? That’s a philosophical question we can’t answer, but if you want to exclude Le Shuttle, there were 10.3 million Eurostar passengers in 2017 and roughly 7.5 million of them were going to or from France, bumping it down to third place.

Next is another international sea link: Denmark-Sweden over the Øresund Bridge (made famous by Scandi noir detective series The Bridge). As well as long-distance trains, the bridge also has a busy commuter service between Copenhagen and the nearby Swedish city of Malmö. Running every 10 minutes each way at peak times, it must be the most frequent international rail service on the planet.

France-Belgium isn’t a single rail connection – instead, it’s a half a dozen or so lines ranging from high-speed TGV to tiny local services. In these statistics, the two are counted equally, so each passenger could be a diplomat zipping first class from Paris to Brussels, or it could be a worker in Kortrijk commuting to their job in Lille in the morning.

For Germany-Switzerland, most of those 6 million are definitely commuters. The Swiss city of Basel, the French city of Saint-Louis, and the German city of Lörrach all lie right next to each other and form an urban area called the “Trinational Eurodistrict”. These share a suburban train network that crosses the German-Swiss border three times and the French-Swiss border once. 25 million people use this Trinational S-Bahn every year, with about 10 million of those using the international sections. Not everyone on those sections will have crossed into Switzerland (some will just gone from one German or French station to another) but it will still be responsible for a big part of the total passenger count for Switzerland.

A commuter train from Basel, Switzerland to Zell, Germany. Not all international voyages are glamorous. Photo: Wladyslaw Sojka/Wikimedia Commons

Rounding out the top five is France-Luxembourg. With 4.3 million passengers a year, this pairing is twice as busy as France-Germany – how, when Luxembourg only has half a million residents total?

Yet again, commuters are to blame. Luxembourg is a very expensive place to live, but its capital city is only a few kilometres from the border. Why not get a house in French Thionville and take the train to work every day instead? If just 10,000 people did that, in a year they’d cross the border 5 million times.

This is the secret to racking up a really high passenger count. You don’t need that many people crossing the border as long as they do it every day. International train journeys aren’t always glamorous long-distance adventures – a lot of them are regular commutes. Compared to an Øresund train every 10 minutes or a Le Shuttle every 15 minutes, intercity trains that only run once every couple of hours have little impact on the total passenger count.

Passenger trains that cross borders are overwhelmingly a European thing – it’s only in the European Union that you get the combination of small countries, wealthy economies and open borders that makes international rail travel possible. There won’t be many non-EU routes that can compete.

France-Monaco is one. Like Luxembourg, Monaco is tiny and extremely expensive, so a lot of people commute in from France. Train travel in Monaco (not technically an EU member state) is kind of weird, since it only has one station (operated by the French SNCF). There were 7.2 million people using Monaco-Monte Carlo station in 2017, and every single one was an international passenger.

Not all of them will have been French travellers – there are regular trains to Italy and occasional trains all the way to Russia – but a bit of maths says there must still be well over 6 million people going between Monaco and France, putting it safely in fourth place.

Belarus-Russia might deserve a place on this graph. OSJD (the railway authority for Eastern Europe and Asia) has published data saying that 3.7 million passengers travelled by train internationally in Belarus in 2017, but annoyingly it didn’t release where they were going. Belarus borders five different countries, three of which are EU members.

We know from the EU data that about 500,000 passengers went between Belarus and Latvia, Lithuania or Poland, which leaves 3.2 million to split between Russia and Ukraine. Most of these probably went to Russia, but exactly how many is impossible to say. (Please get in touch if you know otherwise.)

Russia and Ukraine would also have made it easily a few years ago – 5.1 million took the train between them in 2013 – but passenger numbers have crashed in the wake of the occupation of Crimea.

This is all still inside Europe though. Are there any international train connections on the other continents that can compete?

South America has a broken-up rail network with very few border crossings. Africa has the remnants of a colonial network more focused on transporting minerals and fruit than people. Australia doesn’t have any borders to cross.

North America? Although freight trains run between the US and Mexico, the US-Canada border is the only one on the continent crossed by passenger services. The US counts every single person who crosses its borders and – assuming the same number of people also went the opposite way – that adds up to 0.57 million passengers crossing by train.

Except…

The busiest US/Canada railway border crossing was at Skagway which saw about 190,000 passengers. You’ve probably never heard of Skagway because it’s a tiny village in Alaska. But it’s also got a cruise ship port and a narrow gauge heritage railway that goes into Canada, which authorities consider a border crossing. Yes, a tiny tourist train in Alaska had more passengers than any main-line international service in North America. That’s pretty sad.

(In case you care: the busiest proper railway was the Seattle-Vancouver line, with about 180,000 passengers crossing the border.)

The busiest international railway line in North America. This photo was taken in 2018. This is not a joke. Photo: Explore1940/Wikimedia Commons

That leaves Asia. There are some very rail-heavy countries in Asia, but international trains are hard to find.

Japan and Taiwan are both islands, so they’re out. North and South Korea are divided by the DMZ. Political tensions mean there are very few services running from India to Bangladesh or Pakistan. China’s population is focused on its east coast, a long way from its borders, and it’s ringed by deserts, mountains and tundra. True, millions of people take the train between Mainland China and Hong Kong every year (a staggering 112 million in 2017), but that’s not a proper international border.

No, for a busy international railway there’s only one place to look: Singapore. Singapore demolished most of its international tracks a few years ago, but a small stub remains linking the island to the Malaysian city of Johor Bahru. A shuttle service runs back and forth over the causeway, and in 2018 it had 2.74 million passengers – as many as all trains between Czechia and Slovakia or Belgium and the Netherlands put together.

If we update our graph (making some fairly rough estimates about France-Monaco and Belarus-Russia passengers) then we get:

  ​

European railways in blue, Asian ones in red.

So there you have it. The busiest international railway connection is France-UK, unless it’s Denmark-Sweden or Hong Kong-Mainland China. They’re almost all in Europe – and definitely not in North America.

Stephen Jorgenson-Murray tweets lots of this kind of nonsense at @stejormur.