There are many stories and traditional tales, especially in the Cornish region, of William flying about from mine to mine in a steam chaise lit by gas. 

The inference of a full scale carraige, some ten years after the first model was started, is clear, if almost impossible to prove. But there are two references which suggest that William did indeed build a large carraige, most probably at Budge's Tucking Mill Foundry, which was seen by the inhabitants of Redruth. 

DOCUMENTED EVIDENCE :

1815, May - John Murdoch (William's son), writing to James Watt Jnr

In the main text of the letter 

"The model of the wheel carraige engine was made in the summer of 1792, and was then shown to many of the inhabitants of Redruth - about two years after Trevithick and A.Vivian called at my fathers house in Redruth to consult him about removing an engine then on Hallamanin mine to Wheal Treasury Mine, where they wished the engine to act double. My father mentions this circumstance merely to bring to their recollection that on that day they asked him to shew them his model of the wheel carraige engine which worked with strong steam and no vacuum. This was immedietly shewn to them in a wroking state.

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Note : The Trevithicks were next door neighbours to the Murdoch's. 

"They (Boulton & Watt), also say that driving a carraige was their invention; that their agent, Murdoch, had made one in Cornwall, and had shown it to Captain Andrew Vivian from which I have enabled to do what I have done."

"I had desired Captain A. Vivian to wait on you to give you every information respecting Murdoch's carraige, whether the large one at the foundry was to be a condensing engine or not. Is it possible that this engine might be burnt (burst) by gas?"

These references to a larger carraige no doubt are what gave rise to the steam chaise stories of Murdoch and his gas lit carraige. However, it is highly unlikely that William used such a vehicle as a day to day run-about simply because the topography and the poor state of Cornish Turnpike roads in the 1790's would have made it practically impossible. 

What is more likely is that William gave demonstrations where space allowed and in all circumstances probably only at Budge's Tucking Mill Foundry. "Old Budge" who was financing Murdoch's carriage building was by now nearly 70 years old, and quite possibly lost interest in the scheme as Williams idea's started to become more widely known and copied. 

By the time he had written his letter to Giddy in 1803, Trevithick had made a trial run with his own carraige on December 24th 1801. Trevithick's "Dragons" were a local spectacle for two years and were also seen on the streets of London for a short time during 1803. 

William himself lost interest for he had been for some time conducting his own gas experiments. In the same year that the Tucking Mill prototype was built, William had invented and perfected his "gas lighting" apparatus. 

Because of this it is highly possible the last carraige had some sort of gas application to it. Whether it was powered (burnt by gas), or the boiler pressure tested (burst by gas), we simply do not know, but William did use pig bladders inflated with gas, which were covered in pin holes, which the gas escaped through and which could be lit. Two of these "gas lanterns" mounted on the carraige are where the stories of the "gas-lit steam-chaise" originate from. 

What became of The Tucking Mill prototype is unknown.