The railway's offer to customers (which is the basis on which the railway and the customer agree a contract) is laid out in the timetable.
The timetable is undoubtedly part of the contract, correct. But it's not the sole basis of the contract. What it is, is the times at which the consumer has the right to demand carriage (be it by train or by alternative transport). The right of carriage is the consideration by the railway, not the timetable, and the geographic right of carriage (by way of the permitted routes) is precisely defined by the provisions of the NRCoT and the Routeing Guide.
The timetable specifically shows that no train is offered at the given time for customers to travel from Wycombe to Marylebone.
It shows that the consumer doesn't have the right to demand carriage at that time. But if a train does turn up, opens its doors and the consumer gets on, then they are doing nothing wrong as long as they travel within the permitted routes of their ticket.
Since no service was offered, no contract can have been formed.
Err, no. A contract has been formed, which gives the consumer the right to insist on carriage at the times shown in the timetable. But they also have the right to travel in any other train that follows a permitted route, which we can be certain a direct train from the origin to the destination stations printed on the ticket will be (as there are no relevant route restrictions).
The published timetable shows that the train in question isn't available for the journey made.
It shows that the consumer doesn't have the right to insist on carriage at that time. But the consumer has every right to board the train as it is following a permitted route.
Further, it would be nonsensical for the railway to offer a ticket for sale for a journey that couldn't be made.
It might be nonsensical but that doesn't stop ticket machines and some other sources from selling tickets for journeys that can't possibly be completed within the bounds of the validity of the ticket. That it's nonsensical doesn't mean that the consumer is somehow in the wrong or at fault as a result.
So the customer can be taken to know that they had no valid ticket for the train in question.
For as long as ticket-inspecting staff aren't even told of the existence of the Routeing Guide and hence have no way of accurately determining permitted routes, customers can't be taken to know anything either. The man on the Clapham Omnibus wouldn't think his ticket is invalid to board a train just because it isn't in the timetable he studiously inspected.
It is clear that, as the NRCoT make no reference whatsoever to "set down" or "pick up" restrictions, consumers cannot be penalised in any way if they travel in accordance with their ticket's permitted routes, even if it is in violation of a restriction the train companies would like to have apply. There is no contractual or other basis on which to lawfully charge anything extra or to prosecute the passenger. If the train companies want to stop consumers from doing it then they need to physically prevent the consumer from boarding or alighting.
I suspect that outside of the rail enthusiasts' bubble, "normal" people would consider it madness to even
think about penalising passengers in these circumstances, and would consider Chiltern's actions in this case simply yet another example of the rotten practices the industry likes to indulge in.