Clearly Thameslink Only tickets remain valid on the train company of that name.
There is no Train Company called "Thameslink"; that is a service sub-brand (OK, it's "ThamesLink", but that doesn't matter as on a ticket it'd be printed "THAMESLINK" anyway, would it not?).
There is one called "Govia Thameslink Railway" or "GTR" for short, but that is not the same thing. That name is never shortened to simply "Thameslink" and so I disagree with the above interpretation, and would expect a Court to do the same.
The new wording clearly legitimises sub-brand routeing, and this is what is clearly happening here. It is totally clear, now, what a ticket routed "Thameslink" means, and it is totally clear that that does not include trains branded "Gatwick Express". The NRCoC did not permit GTR to have that condition, the NRCoT does. So to me, that is the end of the dodge, and I'm sure a prosecution, if one is attempted, will be successful.
The only get-out I can see is that the clarification is in one of the supposedly non-contractual information panels. As mentioned I think this actually
would be seen as contractual by a Court (though I would be interested to see
DaveNewcastle's learned view on this) and that the "weasel words" are superfluous.
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If that was correct i hink that difficulty could come for passengers who don't know specific routes or lines. If a passenger knows that two trains run between A and C, but that one stops at B and the other doesn't, how are they to know whether the non-stopper is following the same route as the stopper or a different route and so whether or not a zonal/other ticket combinwtion split at B is valid?
If a passenger wishes to use a split ticket, it is up to them to correctly inform themselves of its validity, by asking staff if necessary.