There are some easements (Section E) that specifically mention Tackley and a Routing Point easement which came into force last month; whether that would have an effect I don't know.

I know there are easements coded in journey planners that are not mentioned in that document, so you can't always be sure you've got all the pieces of the puzzle (I won't mention them explicitly in case they get fixed as they are useful).

 
... but are any of your "via Didcot"s actually not via Didcot (station), neither passing through nor calling at the station, but rather taking the avoiding curve? 

 
My comment (posted personally, not as the official view of the Coffee Shop forum) is that you have highlighted here the ludicrous inconsistencies in the Routing Guide.

(Declaration of interest: I have met Trowres several times, over many years, and had the pleasure of working with him on campaigns such as promoting rail services at Dilton Marsh).

CfN. 

 
Thank you, Fourbee, for bravely floating a response. I think that you'd agree that the Routeing Guide is not altogether explicit on the treatment of vias.

Here then, is my initial investigation that caused me to ask the question.

My starting point is the National Rail Journey Planner (JP) and the journey is WESTBURY-OXFORD ; OPEN RETURN.
(The Open Return bit was what I needed, but conveniently avoids the clutter / confusion of advance fares).

FACT1
The JP offers journeys via Melksham at the NOT VIA READING fare and journeys via Reading at the NOT VIA LONDON fare.
Journeys via Bath have been suggested, but with "We have been unable to find a valid fare. Please re-plan your journey".

INVESTIGATION 1
Perusing the Routeing Guide (RG), both Westbury and Oxford are routeing points. The only valid routes specified are using map RB (described as "routes via Reading to Birmingham").
A look at map RB shows that Bath-Swindon-Didcot-(Oxford) is one line on the map, but it doesn't go through Westbury.
There is a separate line on RB that goes Castle Cary-Westbury-Reading(-Oxford).

So Westbury-Reading-Oxford is the only valid route specified in the maps

Westbury-Melksham-Didcot-Oxford is a valid route, I presume, because it is the shortest route.

Westbury-Bath-Didcot-Oxford isn't valid, because it is neither a mapped route, nor the shortest route.

All of the non-advance fares for WSB-OXF are either "not via Reading" or "not via London", so we cannot see any other options that would allow travel via Bath.

In the interests of furthering this investigation, let's try Westbury to Tackley (a station north of Oxford, for which Oxford is the relevant routeing point in the RG - this means that for RG purposes, the same map is used for WSB-TAC as for WSB-OXF).

FACT 2
The JP offers the following:
0705 Westbury via Melksham and Didcot. Fare £138.10 (anytime return route DIDCOT PARKWAY)
0909 Westbury via Reading. Fare £94.90 (SVR return route NOT VIA LONDON)
1112 Westbury via Bath and Didcot. Fare £73.20 (SVR return route DIDCOT PARKWAY)
1512 Westbury via Bath and Didcot. Fare £73.20 (SVR return route DIDCOT PARKWAY)
1554 Westbury via Reading. Fare £71.20 (SSR return route NOT VIA LONDON)
1646 Westbury via Reading. Fare £71.20 (SSR return route NOT VIA LONDON)
1912 Westbury via Bath and Didcot. Fare £73.20 (SVR return route DIDCOT PARKWAY)
2116 Westbury via Melksham and Didcot. Fare £71.20 (SSR return route NOT VIA LONDON)


INVESTIGATION 2
So, we now have fares offered for the route via Bath. But why?
For WSB-TAC, the relevant routeing points are WSB and OXF, so the same routeing restrictions would apply as for WSB-OXF.

BUT...
Whereas for WSB-OXF, the fares are either NOT VIA LONDON or NOT VIA READING (and no others),
for WSB-TAC, the fares are either NOT VIA LONDON or VIA DIDCOT PARKWAY.

Taking the trains in turn:
0705 WSB: This would be valid as the shortest route. Fare offered is cheaper than the NOT LONDON alternative and route does go via DID so all OK

0909 WSB: This is a valid route in the RG. The £94.90 offered is the cheapest valid SSR fare, as the £73.20 VIA DID fare is not valid until 0930.

1112 WSB: Valid via Bath! The fare quoted , £73.20 is the SVR via DID. There is a cheaper SSR fare, which is valid time-wise, but the route restriction is NOT VIA LONDON.

1512 WSB: Same as 1112 WSB.

1554 WSB: The journey is via RDG. This attracts the lowest fare, the £71.20 SSR route NOT VIA LONDON.

1646 WSB: Same as 1554 WSB

1912 WSB: Same as 1112 WSB

2116 WSB: Via Melksham, this attracts the cheapest fare, the £71.20 SSR Route NOT LONDON.

CONCLUSIONS
1. The VIA DIDCOT is forcing some sort of route analysis that is not just a filter of the list of valid routes for the end-to-end journey.
2. Because the VIA DIDCOT option is not offered as an SSR (Super-offpeak), the fares presented by the JP can be counter-intuitive.

I have not established yet whether or not the JP is analysing WSB-DID and DID-TAC as two separate routes specifically for the VIA DID fares, though this would seem to be logical.

Any comments so far?

 
My initial reaction was it would be b) from the original list of options based on what I thought I had read in the past.

Going to https://www.nationalrail.co.uk/travel-information/routeing-guide/ and reading Section A, in my view, the via point on a ticket can restrict the choice of routes to a subset of permitted routes (i.e. via the via point). That means a) of the choices in the original post. It is mute on the situation where the via point is not included in the permitted routes.

Reading Section F, it includes an example of a fare routed via London and that correlates closely with b) of the choices in the original post.

Overall then, c) is the closest match IMO; having said that the Consumer Rights Act mandates that if a term is unclear it must be interpreted in the way most favourable to the consumer.

Just my 2p/interpretation. YMMV!

 
Hard to work out without station names.

What's the routeing printed on ticket?

Otherwise its a rather convoluted academic exercise that may well not give a definitive answer.

Plus, I've got cricket to watch!

 
Cue Justin. 

 
Here is a question to test Routeing Guide experts (or anyone who would like a challenge):

I wish to make a journey from station A to station B. Both of these are routeing points.

The Routeing Guide lists only one valid route between A and B, although two other "reasonable" routes exist.
 
If there is a ticket from A to B with route "Via C" (where C is common to all three possible routes), how are valid routes from A to B via C determined?


Here are possible options:
a) The "via" could only filter out routes from the list of valid routes, so in this case there would still be one valid listed route (plus the shortest route)

b) Valid routes would be determined separately for A to C and C to B

c) Something else.

Anyone know the answer?