In message , at 17:00:06 on
Thu, 29 Sep 2011, Paul Scott remarked:
In fact, now I look closer, you don't need to do the BoJ trick, because a
FP-Shelford ticket is the same price as a FP-Cambridge one (ie the final
leg south is free). It also means that if you buy the Shelford ticket it's
possible to return via Cambridge as well as arrive that way.

Not according to the NREs OJP - as I wrote earlier it is putting up the
'multiple tickets' warning for journeys via Cambridge.

Their algorithm/database doesn't seem to believe in the "Any Permitted"
ticket also being usable via Cambridge, that's the problem. It is
permitted for the reasons I explain below. FWIW the FCC ticket sales
site also doesn't offer the Any Permitted ticket via Cambridge.

How/where are you getting your info?

The [offpeak day return] fares from (an admittedly out of date) Avantix
CD, the routing from the Routing guide, with a Finsbury Park to Shelford
journey having routing points at Finsbury Park and either Cambridge or
Stansted.

Using the "fares rule" [Cambridge ticket doesn't cost more than the
Shelford one] I can opt for Cambridge as the routing point, which offers
up Map WA. (Finsbury Park to Stansted is routed only via London).

Map WA shows the two London lines up to Cambridge, so Shelford to
Finsbury Park has a mapped route via Cambridge.

Looking at the latest prices on the Eastcoast site (which is vastly
easier to understand than the NationalRail one):

Offpeak Day Return Finsbury Park to Cambridge £24.50 "Any Permitted".
Finsbury Park to Shelford £24.50 "Any Permitted"

Typical timetable for the latter northbound on the KGX line:

Finsbury Park 10:11 FC
Cambridge 11:29
Train terminates at Cambridge

Cambridge 11:51 LE
Shelford Cambs 11:56
Train continues to London Liverpool Street

Or northbound on the Liverpool St line:

London Underground from Finsbury Park to Tottenham Hale
Please check operating hours for this service

Tottenham Hale 11:11 LE
Shelford Cambs 12:14
Train continues to Cambridge
--
Roland Perry