The ATOC Internal guidance being referred to in a number of posts is available on the ATOC website for all to see, here :
http://www.atoc.org/download/clientfiles/files/ATOC%20CoP%20-%20Passenger%20information%20during%20disruption.pdf
If you read this document it does not say that mutual ticket acceptance is automatic for one cancellation. In fact it does not mention about what happens with one cancellation, primarily because that isn't considered to be disruption.
Ticket acceptance between TOCs and routes comes as part of the enhanced customer service offering once CSL2 is declared by a TOC. The threshold for declaring CSL2 varies from one TOC to another, based on a number of factors as outlined in the above document.
"Each Railway Undertaking should define its Service Disruption Threshold/s above which
these PIDD arrangements will apply..." Once TOC A has declared CSL2 then mutual ticket acceptance is automatic on alternative TOCs or routes, as defined in each TOCs plan.
"The declaration of CSL2 allows a wider understanding of the issues throughout the industry, notably amongst other Railway Undertakings who are able to provide assistance, and is accepted as the trigger required for additional assistance." At this time a pager / tyrell / email message is sent out to all affected staff at all affected TOCs who will then be aware of what is happening and accept tickets accordingly. I must get at least 1 message a day on average in this regard. So mutual ticket acceptance is not automatic until "disruption" is declared and CSL2 is in place, and at what stage this is declared varies according to each TOCs plan. The comments about all TOCs working together and treating all passengers to the same high standards journey clearly meant to apply when CSL2 type incidents occur and everyone pulls together.
If one train is cancelled or until such time as Disruption / CSL2 is declared, then unless TOC A asks for ticket acceptance from TOC B via the same or alternative route, then there is no automatic ticket acceptance in place. Now, it is extremely common for TOCs to request such arrangements on a one off basis, and some of these messages are received each day too. It is also not uncommon for staff at a station or on a certain train to endorse tickets to authorise travel - I see issue such endorsements regularly.
In respect of the Advance Tickets Ts & Cs on the NRE website, it says
"When and where the ticket can be used
Tickets are valid ONLY on the date and train service(s) shown on the ticket(s).
Where applicable, you must travel in the Class and reserved seat(s) shown on the ticket(s).
Tickets may only be used on the services of the Train Company (or geographic route where applicable) shown next to ‘Route’ on the ticket.
If the ‘Route’ also states ‘and Connections’, travel is allowed on appropriate connecting trains where shown on the ticket(s) or other valid travel itinerary."
So it highlights you must travel on a certain train, in a certain class and possibly a certain seat, and only on a certain train company or route.
Further down it says that if you are delayed you can take the next available train, or if your train is cancelled arrangements will be made to accommodate you on another train.
It does not say whether these comments over-ride the conditions noted further up the same page (namely the route and TOC restrictions). It could be that "available" in this case means the next "available" train is a train run by the same TOC as noted on the ticket. Nor does it say what the "another train" is in cases of cancellation. It is my opinion and interpretation that unless at some point the relevant authorisation is given, then you must follow the route and TOC restrictions on your ticket. This authorisation may be a blanket relaxation when CSL2 is declared, or a one off relaxation agreed locally or individually. I am aware however that others disagree with that view.
NOW, I am not saying that in the OP's case the situation warranted an extra charge being levied, I feel I don't have enough information to judge that.
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Why is that a surprise? Or something to be ashamed of? The point of the industry, the reason you are all there is to get passengers to where they want to be. People demand good service at low prices, it's how the market is supposed to work to serve them that, not just moan that they want it and charge them more because you can (which, forgive me, seems to be the approach you most often endorse). A fair deal can mean that no cake and eating need take place. But if there is, the customer gets to have their cake and eat it. You don't. Deal with it.
Good service at low prices is perfectly possible, and the market generally does work well to deliver that. However, as it stands at the moment, low prices come with some restrictions, often a TOC or route restriction. And while the industry does work well together to get people around the place and where they want to be, if you have a cheaper route or TOC specific ticket then you must accept they come with some restrictions, and the waiving of those restrictions is not automatic at all times under every circumstance. Different products are offered at different price levels to suit the needs of different travellers. If people wish to have greater flexibility, then there is normally a higher priced ticket which gives them that. The argument about whether that flexible ticket is too expensive is an entirely different one altogether, and I agree that many of them are, but that is the way it is.
Many people having used the airline comparison, I will use the bus industry comparison. A route near me has 2 or sometimes 3 different operators running along it. Different fares are available. Each bus company issues it's own operator specific single or day tickets, or there is the MetroDay which is more expensive but can be used on all operators. If I buy a Transdev Day Ticket and the Transdev bus does not turn up I cannot just get on the next First bus that comes along and expect them to accept my Transdev ticket. If I purchased the more expensive Metroday, then I could, without problem. The choice is mine as to which to buy in the first place.