Perhaps this will lay the issue to rest. They were _based_ on extensively modified Marklin Gauge 1 models. I assume the word of people who actually built and operated the models is acceptable?
See:
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"To obtain a satisfactory size and the necessary level of reliability, the model-makers worked in Gauge 1, using Marklin components as a basis for the locomotives. In fact, the Marklin items are barely recognisable, being confined to the chassis (often extensively modified), motor and wheels. The chassis had to be anglicised and new bodies constructed to represent the various characters from the books. In all, seven locomotives are featured, Thomas and Percy, the two small mischievous tanks, Gordon, Henry, Edward and James, and of course Toby the tram engine."
Also
"Apart from the locomotives, the model-makers had to provide a wide range of rolling stock, most of which was scratch-built using parts from the Tenmille range of gauge 1 accessories and fittings. Two other important characters are Bertie the Bus and Terence the Tractor, the latter quickly nicknamed Drac the Trac by the film crew."
The site includes a number of photos showing both models and people which make it rather obvious they are NOT O scale. In particular, see the photo captioned "Set dressing 1984 - filming of Tenders and Turntables/Breakdown Train"
Further down there is a quote from an article that theorizes that the engines were made from O scale locomotives, and later remade from Marklin... but there are no quotes or cites to support that contention. I find the article quoting the actual builders of the models far more reliable.
See also:
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