We could guess, or we could interrogate the actual data that's been provided in a Freedom of Interest request. It's amazing what questions TfL will answer these days.
Hey presto, a spreadsheet with 65536 lines of data duly ranked by number of passengers.
To be clear this is a) journeys in the calendar year 2021 b) from one tube station to another c) made using Oyster or contactless
And the 20 most popular tube journeys are...
Most popular of all, by some distance, is Brixton to Oxford Circus. The journey people most want to make is from the end of the Victoria line to the heart of the West End. 594486 passengers annually equates to about 1600 passengers a day.
In second place is the same journey in the opposite direction. Intriguingly 78,000 fewer passengers travelled from Oxford Circus to Brixton than from Brixton to Oxford Circus, a 13% difference. That doesn't mean they didn't travel back to south London, just that they probably went via a different route.
Next up is Victoria to Oxford Circus, a shorter version of the top journey. Oxford Circus appears as the entrance or exit station for half the journeys in the top 10.
Then it's North Greenwich to Canary Wharf and back again. It's only a one stop journey but it helps a lot of south London residents get to work in Docklands. For comparison, that's about two thirds of the number of people who cross the Thames by cablecar.
Half of the top 10 journeys are either to or from a National Rail terminus, confirming the importance of the tube for feeding people into the centre of town. Rail routes from south London predominate because there's less tube down there.
The top 10 journeys are all on either the Victoria line or the Jubilee line. It's probably not a coincidence that these are the two most recent tube lines, deliberately planned for optimal connections.
It takes until 13th place until a journey on another line appears, that's Liverpool Street to Oxford Circus on the Central line. The Piccadilly line squeaks into 17th place with Leicester Square to King's Cross.
It shouldn't be a surprise that all of the top 20 journeys are on a single line. Although many journeys tend to require an interchange somewhere, the journeys with the most passengers are going to be those that are the simplest to make.
Indeed all of the top 99 journeys are on a single line, with the Jubilee line by far the most popular.
• Jubilee line 37
• Victoria line 26
• Central Line 17
• Northern line 7
• Piccadilly line 5
• Circle line 4
• Bakerloo line 3
The Circle line's top journey is Liverpool Street to King's Cross (in 30th place) and the Bakerloo line's top journey is Waterloo to Oxford Circus (in 86th place). It takes until 108th place for the Waterloo and City line to appear, although it should be noted that the line was closed for half of 2021 and had restricted opening hours for the remainder. The District line doesn't register in the list until 161st place, which is Embankment to Tower Hill.
The 100th most popular journey is the first to require a change of trains. It's Barking to Stratford (which is also the first appearance of zone 4 on the list). However this journey can also be made by direct c2c trains at weekends so maybe that's a significant part of what's being counted.
The first genuine journey to require a change of trains is King's Cross to Waterloo in 133rd place. It sounds like the sort of journey you ought to be able to make on one train, but you can't. That's probably why TfL used it for their 2019 case study into passenger routing using depersonalised wi-fi data (in which they discovered that 32% of passengers change at Oxford Circus, 27% at Green Park and at least 15 other routes are used).
The second genuine journey to require a change of trains is Whitechapel to Stratford, way way down in 231st place. When the data was collected this required a change at Mile End but today it can be made in one stop via Crossrail. It'd be interesting to know how this'll affect future statistics.
What you may be thinking at this point is "OK, but what are the least popular journeys?" Unfortunately the data doesn't go down that far, it cuts out at journeys made by just 31 passengers over the course of a year. These include unlikely trips such as Northwood to Mill Hill East, Caledonian Road to Roding Valley, Ickenham to Chalk Farm, Wanstead to Eastcote and Hendon Central to Upminster Bridge.
So this is not the "amount of people that done every possible route in the tube". If you know your maths you'll have been suspicious from the start. The total number of possible journeys between 272 stations is 73712 but the FoI spreadsheet only has 65536 rows. This means 8000 journeys don't appear, and we may never know which of these had the fewest number of passengers or even none at all.
But if you know your spreadsheets you'll have been thinking something else, which is "ah yes, Excel 2010 spreadsheets have a maximum of 65536 rows." It sounds like whoever compiled the data for the FoI request, or whoever readied it for public consumption, was using a spreadsheet program that's at least 12 years old. TfL aren't wasting your fare money on up-to-date software, which you may or may not find reassuring.
It's important to remember that 2021 wasn't a typical year, it was still very much affected by lockdown restrictions and advice not to travel. We know from other data that this had the effect of reducing travel in zone 1 and boosting it elsewhere, which makes it all the more impressive that the top journey was to Oxford Circus. So take all this pandemic-related passenger data with a pinch of salt... but I bet the underlying trends hold up.
Thursday update: The FoI response has been deleted and then replaced with a new spreadsheet containing all combinations down to the 288 journeys made by only one passenger. These include North Harrow to West Ruislip, Colliers Wood to Wimbledon Park and Chesham to Debden. The new database has 70224 lines of data. Combinations with zero annual journeys are not listed - these include Croxley to Ruislip Gardens and Theydon Bois to Ickenham (in both directions).
