The book 'The Steaming Sixties 4. The Days That Were - The Withered Arm in Cornwall' Irwell Press 2009 , author Peter Coster, has two sections on the Wenford Bridge branch. On 17th August 1962 he travelled on the Wenford goods, and as he had official permission to do so he says a Queen Mary was attached for his use. His photos in the book show it at various locations along the line, including Wenford dries and I think, the terminus.
There is also a photo taken of the Queen Mary at Dunmere Crossing on May 5th 1964.
Treneglos gets a mention in the introduction to the book.
The South Western Circle used have what they called portfolios on various South Western topics, which contained relevant photos , drawings, plans, and articles, not all of which were from the railway press, which it would lend out to members. The one on the Bodmin and Wadebridge line included an article from the company magazine of, I think , one of the car manufacturers. In this the author describes a trip from Wadebridge to to Wenford Bridge. He states that at the time (the 1960s) one could buy a return ticket to Wenford Bridge, and the Queen Mary would be attached to the Wenford goods for your use.
One of our club members is in the Circle. I must ask him if the portfolios are still available.