It's common to refer to 56.5-inch standard gauge as "Stephenson gauge", but he certainly didn't invent it. I don't know if it really goes back to the Romans, but it does appear to have some antiquity behind it nonetheless.
In the 18th and early 19th centuries in Britain, pre-steam, there were two major regional traditions in the construction of horse-drawn tramways for coal mines, quarries, etc. One was in the North of England, where the track gauge was between 4 and 5 feet and wagons were built with a standard capacity of 3 tons. The other was in the West Midlands and Wales, where the gauge was always 3 feet 6 inches or less and wagons were proportionally smaller as well. Examples of these narrow gauge tramways include the Dowlais tramroad at 2ft 8inches and Coalbrookdale at 3ft. Incidentally, it was these Welsh and West Midlands narrow gauge roads that were the first to use cast-iron plate rail (starting at Coalbrookdale in 1767), even while their Northern counterparts were still using wooden rails covered with wrought-iron straps. Also, Richard Trevithick's first steam locomotive in 1803 was built for the Coalbrookdale tramway and so appears to have been 3ft gauge! (Trevithick's better known Pen-y-darren engine of 1804 was not the first.)
All of the the above is discussed in detail in M. J. T. Lewis' article "Steam on the Penydarren" (about Trevithick et al.) that appeared in the
The Industrial Railway Record in 1975.
The early standard gauge common carriers such as the Stockton & Darlington and Liverpool & Manchester came from the Northern 4-5 foot gauge tradition in which Stephenson worked. (Stephenson was a Yorkshireman after all!) Standard gauge therefore begins in the North of England. The earliest narrow gauge common carriers such as the Ffestiniog and Talyllyn of course came from the Welsh/West Midlands tradition. Narrow gauge begins in Wales.
So that suggests the question, if standard gauge goes back to the Romans, does narrow gauge come from the Celts?
-Philip Marshall