I did some more investigation using your fare finder, and some routes shown an alternative via z1 which is cheaper in the evening peak with a railcard (e.g. East Croydon to Elephant & Castle). Others don’t such as Ealing Broadway to Notting Hill Gate where the routing via Paddington using the EL is probably faster than the Central line and I assume would be charges at the z1 fare. Am I right then that approaching a boundary station from the z1 end will trigger a z1 fare? So I could go West Ruislip – Marylebone – Paddington – Notting Hill Gate, or West Ruislip – Queensway (OSI) Bayswater – Notting Hill Gate in order to get the z1 fare?
What happens if I travel West Ruislip to Lancaster Gate touch out and immediately in then return to Notting Hill gate – I assume that would be seen as 2 journeys, would it?