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If I have a ticket from A to B with a route restriction "via C", how is the entirety of valid routes worked out using the routeing guide. Do I use the routeing guide to look up valid routes for A-C and C-B?

If you wish to work on a specific example, how about Bradford-on-Avon to Brighton "via Salisbury" ?

Section F of the routeing guide, when referring to the choice of mapped permitted routes, states that you can use any mapped routes provided ... you don't avoid a station the fares route mandates you travelling via. That suggests the correct process is to look up all the permitted routes from A to B and then exclude those which are not via C. However there are cases where there are no permitted routes from A to B via C. Nowhere that I've found in the routeing guide is this possibility considered, but presumably the only way to determine the permitted route would then be permitted routes from A to C and then C to B. If there are permitted routes from A to B via C then these are the permitted routes for the journey though, rather than the combination of A to C and C to B.

Thanks for your interpretation, Southern Stag. As you say, using the routes for A to B is suggested, rather than stated explicitly. It seems to open up the possibility of making the journey from BoA to Brighton via (say) Salisbury and London on a "via Salisbury" ticket.

This weekend, Salisbury-Waterloo services are being diverted via Southampton. I was intrigued that NationalRail website automatically offered a Salisbury-Brighton "via Southampton"  fare for the roundabout route via Southampton and Clapham Jn.

And the fare is set by... FGW.  ::)

If you want a headache look how many routing options you get for Southampton airport- Exeter st David's

Working out routes from BoA to Salisbury and then Salisbury to Brighton would also permit travel via London, the first permitted route from Salisbury to Brighton Group is London. It could be that the via Salisbury fare is just intended to prohibit travel Westbury/Bath-Reading-London, as otherwise a via Barnham routeing as used on other West Coastway flows would be more suitable to mandate travel Bradford-Salisbury-Southampton-Barnham-Brighton. There is a not via London fare at ^61, but the cheaper ^42 via Salisbury fare permits travel via London, provided you travel to London via Salisbury. It is also valid for cross London travel despite no maltese cross being printed on it, which might lead to problems with it being accepted on routes via London. The routeing guide does clarify that for some long distance fares there will be no maltese cross but they are still valid for cross-London travel.

How are the LU barriers programmed to recognise which tickets can be used? This sounds like a great recipe for an argument with the gate supervisors.