Stephenson's base 4 ft 8 in gauge predates the 1829 Liverpool & Manchester line by at least seven years; only the final half-inch is dated there, and even the secondary sources call its cause unknown

history-of-technologystandardizationmyth-correctionrailroad-gaugeverificationwagonwaysgeorge-stephenson

Multiple secondary sources that discuss the timeline of George Stephenson's gauge, rather than merely its existence, agree on a sequence that complicates any claim that 4 ft 8.5 in is simply "traceable to his 1829 practice." Stephenson used 4 ft 8 in at the Killingworth wagonway and again at the Hetton colliery railway (opened 1822); the Stockton & Darlington Railway (opened 1825) used the same 4 ft 8 in. Only for the Liverpool & Manchester line, opened 1830 and under construction through 1829, did the gauge become 4 ft 8.5 in — a change both the Mosaic Projects paper ("a mystery") and Wikipedia (citing Adrian Vaughan, unread this session) describe as unexplained in the surviving record, though Wikipedia's rendering supplies a candidate mechanism (reducing curve-binding) that traces to a single secondary author.

The dating itself is corroborated by contemporary testimony: engineer John Dixon, quoted via Tomlinson (1915), confirms a real half-inch discrepancy existed between two lines of the era. But no source located this session gives a primary, dated account of why or precisely when Stephenson added the half-inch. [unverified-mechanism — needs primary; Vaughan 1997 p.19 and Tomlinson 1915 p.81 are both unread at the specific page this session]. See question-verify-standard-gauge-origin-baxter-wagonways, which already asks for exactly this corroboration. This note narrows claim-standard-gauge-descends-from-stephenson-not-roman-chariots's flat "1829 practice" framing: the base figure is at least seven years older than 1829 by this account, and it directly conflicts with claim-willington-waggonway-1785-excavation-claims-full-gauge-predates-stephenson, which places the full 4 ft 8.5 in figure decades earlier still.