The Out of Station Interchange, or OSI, is a TfL kindness which allows you to exit one station and enter another nearby without being charged extra. Only certain stations are included (a full list is here/here), and the transfer must be completed in a set amount of time.
For example if you arrive at Hammersmith on the Circle line but want to change to the District, the OSI permits you 20 minutes to cross the main road, maybe buy a coffee, even nip off to the toilet, then continue your journey for no additional fare.
In a recent FoI request, TfL have revealed how many times each of the 130 OSIs was used during a four week period spanning either side of Easter. This is of course extremely interesting, and can be used to list...
The 10 least used OSIs (11th April - 8th May)
0) Hackney Downs ←→ Hackney Central (20 mins) (0 passengers)
Absolutely nobody uses this OSI because it's now obsolete. A 400m walkway connection opened between the two Overground stations in 2015, so nobody now needs to walk out of one set of ticket gates and back in at the other, so nobody does. Let's disregard this one.
1) Marylebone (NR) ←→ Paddington (NR) (40 mins) (9 passengers)
This is the least used OSI that's actually used. Pay as You Go passengers changing between the rail termini at Marylebone and Paddington can do so free of charge so long as they complete their change in 40 minutes (and don't nip onto the tube or a bus inbetween). The walk's three-quarters of a mile, which explains why hardly anybody does it - only nine people over a four week period, which is roughly two a week. For comparison, on a typical day about 500 Oyster users walk between Marylebone and Baker Street, and about 2000 cross the concourse between Marylebone (rail) and Marylebone (tube).
2) Euston (NR) ←→ King's Cross (NR) (40 mins) (42 passengers)
The second least used OSI is also between two mainline termini. Again it only relates to Oyster or contactless users switching between National Rail services (for example an Overground journey into Euston followed by a slow train out to Alexandra Palace), and doesn't include long distance travellers. Another row in the spreadsheet shows that Euston to St Pancras is a bit more popular (972 passengers), but still not very popular.
3) Sudbury Hill ←→ Sudbury Hill Harrow (20 mins) (46 passengers)
Sudbury Hill's on the Uxbridge branch of the Piccadilly line and Sudbury Hill Harrow on the Chiltern line out of Marylebone. The two stations are barely 100m apart so this could be a well used OSI, but instead it's scuppered by Sudbury Hill Harrow only having an hourly-ish service on weekdays and no trains at weekends. Could do better.
4) Tower Gateway ←→ Fenchurch Street (→ 40 mins, ← 20 mins) (55 passengers)
Pop out of Fenchurch Street's back entrance and you can be at Tower Gateway in under five minutes. The reason almost nobody does is that anyone wanting to change between c2c and DLR probably did it back at Limehouse where the two lines share a station, so there's no point doubling back at Fenchurch Street. Note that passengers heading towards Fenchurch Street are allowed twice as long to make their connection as passengers heading towards Tower Gateway, because OSIs are strange like that.
5) Walthamstow Central ←→ Walthamstow Queens Road (25 mins) (67 passengers)
Walthamstow Central to Walthamstow Queens Road is a recommended connection, ever since a connecting path was opened in 2014, so why are passenger numbers so low? The reason is that Overground users at Walthamstow Central have been counted separately from tube users using the Victoria line ticket gates, and the two streams are highly imbalanced. In fact 3357 Victoria line passengers changed between Walthamstow Central and Walthamstow Queens Road, and that's where the real OSI traffic is.
6) Embankment ←→ Waterloo East (→ 40 mins, ← 25 mins) (80 passengers)
Here's another 'but why would you do that?" Every mainline train which stops at Waterloo East also stops at Charing Cross, and that's a much more sensible place to change trains. What we're probably counting here are people who fancied a nice walk across the Golden Jubilee Bridge and just happened to walk into a station on the other side within the requisite time limit. This number equates to approximately three people a day, which is insignificant in the wider scheme of things. Here's a diagram which illustrates the wider scheme of things, including all the OSIs in the vicinity of Charing Cross and Waterloo.
7) Upper Warlingham ←→ Whyteleafe (20 mins) (89 passengers)
Now we shift to the very edge of the capital, and a sub-five minute walk between the Caterham and Uckfield lines. One reason this OSI is barely used is that Upper Warlingham is the last station within the Oyster zone, so only passengers at Sanderstead and Riddlesdown have any practical use for the switch. Again, about three a day.
8) Clock House ←→ Kent House (20 mins) (128 passengers)
The line from London Bridge to Hayes doesn't meet the line from Victoria to Bromley, and this really is the best place to swap. But it's a half mile walk, and a bit of a local secret, so hardly anybody clocks in.
9) Bromley North ←→ Bromley South (30 mins) (131 passengers)
Two key southeast commuter lines cross near Chislehurst without providing any use place to interchange, so why not try this Bromley switcheroo instead? Mainly because the Bromley North spur has a poor service and, more importantly, stops a full kilometre away from Bromley South. A very decent bus service connects the two stations, but if you board a bus you break your journey and the OSI never kicks in. Only five passengers a day, on average, do the walk and gain the benefit.
10) Canary Wharf (E2) ←→ Heron Quays (15 mins) (182 passengers)
And finally, one of the many possible cross-Canary-Wharf connections. I think we need another map. Figures are daily approximations.
Canary Wharf Jubilee line station has two ends and each is a distinct exit, as you'll know if you've ever accidentally walked out of the wrong one. TfL have therefore created OSIs from each end of the station to each of the neighbouring DLR stations, plus an extra OSI to allow anyone to pass freely from one Jubilee exit to the other. The walk-through is quite popular, officially ticking off around 450 passengers a day. Far more popular are the Jubilee/DLR OSIs at the western end of the station, rating several thousand. Unsurprisingly DLR passengers don't generally head to the eastern end of the station, which is how Heron Quays gets to be the final runt in the least used OSI Top Ten. Heaven only knows what'll happen around here when Crossrail arrives.
And for completeness, The 5 most used OSIs (11th April - 8th May) 1) Waterloo (LU) ←→ Waterloo (NR) (1100000 passengers) 2) London Bridge (LU) ←→ London Bridge (NR) (1070000 passengers) 3) Victoria (LU) ←→ Victoria (NR) (1000000 passengers) 4) Liverpool Street (LU) ←→ Liverpool Street (NR) (680000 passengers) 5) Vauxhall (LU) ←→ Vauxhall (NR) (580000 passengers)
