How can I find out the default route assumed by TfL's Single Fare Finder?

Question

Transport for London (TfL) has a Single Fare Finder which allows you to determine the cost of a contactless/Oyster pay-as-you-go journey between any two stations on the London Underground, Docklands Light Railway, and National Rail stations in Greater London or served by London Overground or TfL Rail. Each journey will return a Peak fare and an Off Peak fare.

For journeys where multiple possible routes exist between the two stations, different fares may be available depending on the route taken (typically when a journey can be made whilst avoiding Zone 1). These are shown in addition to the default fare under Alternative Fares, with a note such as "changing at Vauxhall" to indicate where the fare is applicable. However, since the default route does not have any annotations, how do I determine what the default route should be when alternative fares are available?

For example, if I ask for the fare between Surbiton and Watford Junction, I get five possible fares: 3 variations via Zone 1 (all the same price, but via different stations), one avoiding Zones 1 and 2, and the default fare. However, if I use the Journey Planner for that journey today (27/10/18), all the results route me via Zone 1 - the default fare does not show up as an available option.

Answer 1

One way of checking what the "default" route is is to use the journey planner to plan a journey on a different day, preferably mid-week when there is less likely to be engineering work.

In this case, the "default" route is changing at Clapham Junction to the direct Southern service to Watford Junction via Shepherd's Bush (and so avoiding Zone 1). Today (28/10), however, that line is closed between Shepherd's Bush and Willesden Junction, so it isn't listed as a possibility.

As you'd pass through a ticket barrier at either (or both) Waterloo or Euston today, the system would recognise you'd gone through Zone 1 and so would charge you accordingly. If you were to travel via Richmond and Willesden Junction instead, you'd need to touch on the pink Oyster reader at Richmond to signal to the system you'd travelled that way, avoiding Zone 1 (there is a list of pink readers at https://tfl.gov.uk/fares-and-payments/oyster/using-oyster/pink-card-readers).

Answer 2

The “default” route is simply the fare you would pay if you touch in at the from station, and touch out at the to station without touching an intermediate pink reader. The alternative routes listed will all require leaving and reentering the network (touching out and in again) or using an intermediate pink reader. So there isn’t a single default “route” per se since you can take any route within the network between the two stations.