From trying out journeys on National Rail Enguiries it seems to be permitted for cross London journeys but not for Merstham to London Terminals. Does anyone know why? I can't see an easement.
Just so I know what to say if I am challenged.
This is down to the way the Routeing Guide works.
Merstham is associated with Croydon (a Routeing Point Group), and Redhill (a Routeing Point).
To work out the 'mapped' routes you need to undertake a fares check using NFM64 fares (i.e. from 1996), to see whether the fares from a given associated Routeing Point are higher, lower or the same as those from your actual origin.
Fares from Merstham to London Terminals were lower than those from Redhill to London Terminals, but higher than those from Croydon Stations to London Terminals. Therefore Croydon Group passes the fares check, but Redhill fails it.
However once you go beyond London Terminals, fares are no longer individually priced by station, and Merstham is contained in the same 'flow' as Redhill for most journeys. Therefore, for example, Redhill to Finsbury Park is priced the same as Merstham to Finsbury Park, and hence Redhill now passes the fares check.
When a Routeing Point passes the fares check, you are allowed to travel along the shortest route to that Routeing Point (or a direct train etc.), and then to use the mapped routes from that Routeing Point to your destination Routeing Point. In doing so you might double-back, but that's permitted - this is the reason why we have the fares check, essentially to try and prevent people from using double backs to undercut other fares.
