The terms of an Advance ticket specify some of the special obligations attached to the ticket. Though the ticket simply acts as a token for the obligation the Company has entered into.
What a Railway Company wants from a passenger is the correct fare for their journey. In exchange, the Company enters into that obligation, it expects the passenger to, either: not travel at all, or, to take the exact journey.
I've explained
previously (*) that a passenger can stop short if they regularise the position by buying a single ticket from the destination back to the point at which they wish to alight early. That avoids any complex dispute about what the correct price 'should have been' for the short-Advance, it avoids the question of defrauding the Company of any monies, it avoids any concern about not having contracted for any part of their liability and about the responsibilities of the Company at the stations concerned, it avoids any errors in the demand-management objectives in creating quotas of Advances, it avoids the risk of not holding a valid ticket at any instant of their journey (including the passenger's time on Railway premises).
The only reason I can see why the Company would wish to argue that the passenger travels over the last part of the route twice (doubling-back) would be to enforce the Condition "
You may not start, break and resume, or end your journey at any intermediate station except to change to/from connecting trains as shown on the ticket(s) or other valid travel itinerary.". But as the passenger has already paid for travel (back) to the station at which they arrive, then I'm not able to find any compelling reason to force the passenger to double-back, the passenger who was at liberty not to travel at all, other than the established offences: an attempt to defraud the Company, failure to produce a valid ticket, intent to avoid payment, theft, travelling beyond the destination on their ticket, damages for breach of contract - all of these would fail.
The Company's only remaining argument would be that an additional ticket should be sold, for the actual point to point journey. However, the as tickets held correspond to the point to point journey (and to the actual service: "
Tickets are valid ONLY on the date and train service(s) shown on the ticket(s)."), I would find that an unsupportable claim.
* That was in 2010. I then added
if a passenger holds the tickets A>C via B and C>B and the most onerous Conditions are those pertaining to an Advance for the A>C leg, and the passenger wishes to alight at B without travelling on to C and back, then . . . .
the tickets give the passenger an entitlement to travel, but gives the railway company a compulsion to convey the passenger.
As there is no compulsion for the passenger to travel at all, then the moment that the train arrives at "B" the railway company cannot then compell the passenger for further travel once they have completed their entitlement.
This was argued by many of the knowledgeable members of this forum back then, in this thread:
"Break of Journey" without discussing the Customer Services or media points of view.