Odd ticketing engine result - doubling back

[4COR]

I'm going to be travelling from Caterham to Oxford on a Saturday next month. I decided to search (using a well known site) for fares via Purley, Redhill and the N Downs line to Reading as fancied a different route home.

I put in Redhill as a via for the search and amongst the results got this strange one as follows :
Caterham->Purley, Purley->Redhill, Redhill->Purley, Purley->E Croydon, E Croydon-> L Bridge, cross London then Marylebone to Oxf.

All this was done by on a single off peak ticket - including the double back through Purley. Is this valid to doubleback here? I'd expect some potential hassle between Purley and Redhill if I tried it... (as utterly pointless as the detour seems...).

Thanks

 

[yorkie]

Redhill passes the fares check, so it's valid, though a more sensible use of this routeing point would be to travel via Gomshall!

So it is absolutely valid; you are taking the shortest route to an appropriate origin routeing point (ORP). You are then taking a permitted route (on map LB) from the ORP to London, and a through train from London to the destination routeing point.

 

[56131 EllColl]

If perhaps search via a station on the Redhill Reading line such as Dorking Deepdene then this will give a better result. An obscure route to consider in future is via Whyteleafe/Upper Woldingham and Edenbridge Town/Edenbridge. Of course this necessitates a short out of station interchange twice.

 

[maniacmartin]

I'll expand on yorkie's answer for people who aren't familiar with the Routeing Guide.

I have often thought that Purley should be a Routeing Point in the Routeing Guide, given that it is a junction station where all services from both branch lines call. However, the powers that be felt differently and instead made stations on the Caterham and Tattenham Corner branches be associated with both the Croydon Group and the Redhill Group.

As the fare from Caterham to Oxford is no greater than that from Redhill to Oxford, Redhill can be used as an "Appropriate" Routeing Point for this journey. This amounts to using one of the mapped routes between Redhill and Oxford, and tacking a bit on the end. Booking engines do not implement the additional bit in the NRG in Detail document about the whole journey not having double backs. They only check that the mapped part of the route contains no double backs. Indeed, it has to be this way to allow connections from stations with infrequent services to their Routeing Points, which tend to be larger stations with more services.

Clearly the fares check hasn't produced the intuitive result here as one would expect that only Croydon would be appropriate for journeys via London. Expect a negative easement or similar to appear in a future update now that this anomaly has been exposed.