William Murdoch's work on his first model-steam carraige (the only survivor), came to a halt towards the end of 1784 for various reasons. As intended, James Watt kept William busy "with the cornish business of the day". More importantly, to William at least, was his marraige to Anne Payntor, and within the year his twin sons were born. During this time he quietly worked away on model No2 which had a smaller but more efficient engine. 

DOCUMENTED EVIDENCE :

1786, August 6th - Thomas Wilson, writing to James Watt

"Wm Murdoch desires me to inform you that he has made a small engine 3/4 inch diameter and 1 1/2 inch stroke that he has applied to a small carraige which answers amazingly."

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"I have begun to make a steam-carraige on a large scale but have no hopes of it being useful"

There is no evidence, documented or otherwise that Watt had done so, indeed he contradicts himself in his letter to Black, of October 15th. 

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With Model No2 now built and tested, William Murdoch set off for London to patent his locomotive. He got as far as Exeter where he was intercepted by a suspicious Mathew Boulton who had been informed by Thomas Wilson that William was going to London "for men". 

"I arrived at Exeter on Wednesday night and set out from there on Thursday afternoon to sleep at Oakhampton - about I Mile on this side of Exeter I met 3 of the Foxes going to a general Quakers meeting at Gloscester & in the same Coach was Wm Murdoch. He got out and we had a parley for some time. He said He was going to London to get Men but I soon found he was going there with his steam Carg to shew it & to take out a patent. He haveing been told by Mr W. Wilkn what Sadler had said & he had likewise read in the news paper Simmingtons puff which had rekindled all Wms fire & impations to make Steam Carraiges. However, I prevailed upon him readily to return to Cornwall by next days dilligence & he accordingly arivd here this day at noon, since which he hath unpacked his Carg & made Travil a Mile or two in River's great room*  in a Circle makeing it carry the fire shovel, poker and tongs, I think it fortunate that I met him as I am persuaded I can either cure him of the disorder or turn the evil to good - at least I shall prevent a Mischief that would have been the consequence of his journey to London. I am to dine with Wilson to-morrow and with Murdoch on Monday."

*Landlord Thomas Rivers' "Great Room" at the King's Head in Truro

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"I am extremely sorry that W(illiam) M(urdoch) still busies himself with the steam carraige. In one of my specifications I have secured it as well as words could do it accordingly to my idea's of it; and if to that you add Symington's and Sadler's patents, it can scarecly be patentable, even if free of the general specification in the Act of Parliament; for even granting what I have done to secure it, yet it can act as a prior invention against anybody else, and if it cannot be secured by a patent, for what purpose should anyone labour at it? I have still the same opinions concerning it that I had; but to prevent as much as possible more fruitless argument about it, I have one of some size underhand, and am resolved to try if God will work a miracle in favour of these carraiges. I shall in some future letter send you the words of my specification on that subject. In the meantime I wish W(illiam) could be brought to do as we do, to mind the business in hand, and let such as Symington and Sadler throw away their time and money, hunting shadows."

There is no evidence, documented or otherwise that Watt had drawn, let alone started to build a large steam carraige. 

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" ... you know I have long had plans for moving Wheel Carraiges by steam and I have even described them in one of my patents years ago. I believe I shall make some experiments on them soon but have small hopes of ever making them useful".

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This is the last piece of recorded evidence from the correspondence between Watt, Boulton and Wilson concerning steam carraiges. There is no other mention in any of the letters from this date on. There is also no surviving documentation or letters from William Murdoch on matters relating to the steam carraige. What became of Model no2, which was intended for patent, is not recorded or known but it appears not to have survived. 

There is also some suggestion that it was this model and not Model No1 that gave rise to the "Steam-Devil" story, but with so little evidence either way, a toss of the coin has ascribed it to Model No1, at least until further evidence is found regarding dates and circumstances. 

Model's 1 & 2 may even be the same chassis but with different engines, it would be easily possible, and given Murdoch's nature of making efficient use of matrials, most probable. 

William Murdoch's work on the steam carraige was not yet finished though, for there is more evidence suggesting two more carraiges were built between 1790 & 1792.