Not listed, the viaduct is not addressed at all in the sections of the TWA inspectors report dealing with listed building consent. There were only 3 listed features considered, Ridgmont, Woburn Sands, and Quainton Rd stations.
As long as it doesn't become a listing structure.
I wonder if someone had noticed that there was a separate planning application for the full viaduct rebuild, because the deemed planning permission from the TWA order didn’t include the eventually needed viaduct work? Although that application appears to have been an application under permitted development rights, the local authority would still have been able to comment on the particular visual design of the finished viaduct. But that’s a lot different to a listed building consent application.