Not having any connection with the railway, except as a passenger, it would seem that the RAIB has the correct measured response when adjudicating on unfortunate events like this. The three parties involved are all part of the cause and future prvention of similar incidents:
a) the infrastructure owner - Network Rail. They are responsible for establishing and maintaining a route with a defined structure clearance for the trains they have authorised to pass along it.
b) the train operator(s). They have a primary responsibility for the safety of their passengers, irrespective of their temporal intelligence or behaviour.
c) the general public including those who don't even have a ticket.
The argument that the TOC is predominately responsible is based on the fact that passengers are carried inside the vehicles, and all reasonable steps must be taken to prevent them doing otherwise. A notice on or by a door isn't the limit of
reasonable precautions, as a child or an inebriated person might not have had an adequate warning from it, as the GWR case in discussion proves.
The argument that NR should be able to ensure the structure gauge is kept perfect at all times is specious. Trees are natural growing objects, they get broken in storms and can even move when their roots are exposed, e.g. during exceptional rainfall. Most of the time, the intrusions into the gauge envelope are from twigs etc., which is why there are anecdote posting here. It would be an unreasonable demand on NR to guarantee that these never occur. However, when these minor intrusions do occur, vehicle bodies are strong enough to push them away, - even driver cab windows are generally tough enough to do so without posing an injury risk to the driver. A human head however isn't as strong as steel bodies and a small branch as thin as 1 inch could easily cause a fatal injury.
The responsibility of the TOC to NR includes keeping the train within it's defined loading gauge. Had these passengers been prevented from causing the train to run out of gauge, the report would be a minor damage note at the depot. Thus it would seem that the TOC must carry the major responsibility here, in effect by running a rolling stock design that is not fit for a significant part of its purpose.
Apart from the defensive messages in this thread, presumably from TOC and NR interests, I feel that there is a strong defence by some HST fans here who won't admit that the love of their lives is no longer acceptable on a modern railway unless modifications, (probably prohibitively expensive) are undertaken on all of
them.