A couple of observations, having done quite a lot of reading around this subject during lockdown (but I make no claim to be a historian or academic researcher):
Much of the interest in 'mobile' steam engines around 1800 was actually for use on boats, where weight and size constraints were less.
Quite a lot of surviving 'designs' from that time seem to have been done for patent application purposes rather than actually building things. To reduce the risk of copying or piracy of ideas certain features were often omitted or deliberately slightly misrepresented.
Coalbrookdale was undoubtedly a significant influence on very early metallurgy, steam engine (not necessarily locomotive) and waggonway development but various factors made it a rather unsuitable cradle. Land movement tended to be very short distance, the local waggonway gauge was rather smaller and gradients rather steeper around the sides of the Severn Gorge. So horses could easily pull small waggons, it was very hard to 'miniaturise' a steam engine of the time, there was little economy of scale from longer distance rail movement and local canals already existed.
South Wales offered rather longer distances, had adopted the Benjamin Outram 4' 2" gauge for waggonways and had applied modest gradients.
The North East had had wider gauges, longer distances and cleverly engineered waggonways since somewhat earlier but still used wooden rails and was actually slower off the mark in iron and steel manufacture.
Industrialists like Samuel Homfray (who had interests in turnpikes, canals and waggonways as well as metal production in South Wales) did far more to generally 'push' early railway development than Trevithick.
(All 'IMHO'.)
I appreciate you adding your thoughts to the discussion.
I'm not so sure that there was much of an issue with regard to miniaturising steam engines of the day to make them suitable for railway (or waggonway) usage, at least not in Trevithick's case. He was pretty much leading the field with regard to the use of high pressure steam when most of his competitors were using low pressure atmospheric engines, which meant that his designs were capable of doing the same amount of work from a much smaller and more compact design. That these designs lent themselves to being mounted on wheels to a gauge that suited certain locations probably wasn't part of his masterplan. Reading his correspondence it appeared that he was more keen to show the superiority of high pressure steam and how it can deliver the same amount of work as a low pressure engine but still provide savings in terms of fuel and manpower. How this technology might be applied seems to have been a lesser concern and, although he came up with some novel ideas, very few of them seem to have come to fruition.
Although Trevithick himself talks about his engines being capable of locomotion and even describes some concepts where independent locomotion would be required, he seems to see that as just one of the things they are capable of doing and doesn't appear to have set out to design a locomotive. Indeed, both of the engines (as distinct from locomotives) at Coalbrookdale and Pen-y-Darren were intended to carry out other tasks. The Coalbrookdale engine was initially used to demonstrate it's ability by pumping water up a column of pipes to a height of 35 feet while the Pen-y-Darren engine looks like being a multi-purpose construction as it was intended to test it as described below.
"The waggon-engine is to lift this water, then go by itself from the pump and work a hammer, then to wind coal, and lastly to go the journey on the road with iron." (Letter to Davies Giddy - 4 Mar 1804)
Having provided the Coalbrookdale Company with an engine suitable for locomotion, it would seem that any further development of the engine into a locomotive was undertaken by the Coalbrookdale Company itself independently of Trevithick, as his final mention of it comes in a letter to Giddy dated 22 Aug 1802 in which he says;
"The Dale Company have begun a carriage at their own cost for the railroads, and are forcing it with all expedition."
Due to it's disappearance from Trevithick's correspondence at this point, any further evidence of there ever having been a locomotive at Coalbrookdale would probably only be found in the records of the Coalbrookdale Company.
As to the relative roles of Coalbrookdale, Pen-y-Darren and the Durham coalfields in the development of the locomotive, I think we're talking about the differences between manufacturer and client. Trevithick himself did not build engines but rather designed them and had them built by contractors around the country. Both the Coalbrookdale Company, which was at that time probably Britain's largest producer of iron, and the Pen-y-Darren ironworks were among this throng. Therefore it made sense to test and demonstrate the technologies close to where they were manufactured, the majority of which appears to have been carried out at Pen-y-Darren. Whether this has anything to do with the characteristics of their waggonways or whether due to some other factors (e.g. Trevithick's relationship with the Pen-y-Darren ironworks owner, Samuel Homfray) is open to discussion, but it would seem from Trevithick's correspondence that he spent rather a lot of his time there during this period as well as giving a lot of information and commentary about what is happening there.
However, you are right to point out that these ironworks were perhaps not the best environment in which to develop rail technology, and for that the Durham coalfields were better suited. Quite how this came to be is an interesting topic in itself. Certainly the biggest movers in this arena were the Blacketts who owned and operated the Wylam Colliery with it's 5 mile waggonway linking the pit to the Tyne at Lemington. The first locomotive that they acquired was a Trevithick design and was seen operating in May 1805, although what it was that inspired them to get onboard with the technology at such an early stage is unknown. Certainly it appears that Trevithick had a contractor building engines for him in the area, so no doubt the possibilities were known at that early stage. Unfortunately the Trevithick locomotive proved too heavy for the wooden waggonway and nothing more was done until 1808 when the waggonway was relaid with iron plates. As Trevithick had declined to provide another locomotive, Blackett turned to his own workforce which included William Hedley and Timothy Hackworth (who later became the engineer for the Stockton to Darlington Railway) to come up with their own designs. As an aside, it's known that George Stephenson was born in Wylam and that the waggonway passed by his house. What effect this had on him in shaping his later career is interesting to speculate on, but both he and Hackworth were among the entrants at the Rainhill Trials two decades later.
I think I see what you’re saying about the vagueness of a patent application being a protection against plagiarism, but I would argue that if you want your patent to be defensible it would be better to make it precise so that you can easily show how someone else has infringed. Trevithick fought a number of actions where patents were either challenged or defended so would likely have been savvy enough to know how to find loopholes in others patents and avoid introducing them in his own.
(Information in this post comes from the pages of industrial history website
Grace's Guide which includes Trevithick's correspondence and commentary compiled by his son Francis as
"Life of Richard Trevithick" (1872) and a whole heap of other really interesting stuff.)
