Marshall considers that Gray probably originated in Northumberland in 1810. With Daniel Gooch, John Gray was the first of the early British locomotive superintendents after Hackworth to take an active role in the design, as opposed to the running, of their locomotives. Moreover, in doing so he made his own very definite contribution to mid-century design.
As locomotive superintendent of the Hull & Selby Railway he introduced in 1840 a 2-2-2 passenger engine which, while following Stephenson's horizontal inside-cylinder layout, had radically different framing; the driving wheels had inside bearings, but the carrying wheels had outside bearings. This type had large driving wheels of 6ft, and was further distinguished by Gray's patented valve gear, the so-called 'horseleg' motion, which was the first manifestation of expansion gear. This was also described by David Joy in an article to the Engineer, 1890, 69, 14 February.. This locomotive type later became popular with other railways and designers, the Jenny Lind being the most celebrated example. In several ways Gray was ahead of his time. He correctly rejected the fashionable insistence on a low centre of gravity, and favoured high boiler pressures (up to 100 p.s.i.). After leaving the Hull & Selby Railway he was locomotive superintendent of the London, Brighton & South Coast Railway from 1845 to 1847.
In 1838 he was the first engineer to use the balanced slide valve on locomotives, on the Liverpool & Manchester Railway. In 1839 he applied a form of expansion gear to the Cyclops engine on the Liverpool & Manchester Railway, claiming a 12 per cent fuel economy. This 'horse-leg' motion was used on engines on the York & North Midland Railway in 1840 and in 1842 on the Hull & Selby Railway on which, in 1840, he was appointed Locomotive Superintendent. He was the first engineer to use a long-travel valve motion, of about 6in (152mm), on the H & S in 1840. He was one of the first engineers to discard the idea of a low centre of gravity as necessary for the safe running of locomotivess. On 27 March 1845 he became Locomotive Superintendent of the Croydon, Dover & Brighton Joint Committee, and on the formation of the LBSCR on 27 July 1846 he retained this post at Brighton. During September 1845 he ordered 14 2-2-2s of his design from T. Hackworth at Shildon, but late delivery because of Gray's constant alterations to the design resulted in his dismissal from Brighton in 1847.
Became MIME in 1847, the year the Institution was founded but allowed his membership to lapse in 1852. He died in Bradford on 18 October 1854 in what is described by Marshall as "straightened circumstances" probably due to an action he took against the LNWR in respect of royalties due for the use of his valve gear by the Liverpool & Manchester Railway (see R.T. Smith: John Gray and his valve gear Trans. Newcomen Soc., 50, 139). Daniel Gooch was called as a witness on behalf of the LNWR. Other witnesses included Thomas Cabry, Brunel, George Rennie and William Fairbairn..
Rogers (Chapelon biography) notes that "Comparatively little known, perhaps, is John Gray, Locomotive Superintendent of the Hull & Selby Railway, and yet he was one of the engineers who were was considerably ahead of their time. His 2-2-2 engines of 1840 constituted an advance on several accounts. They had the high boiler pressure for the period of 90lbs per sq in, and he appears to have been the only locomotive engineer of the time who realised that it was not necessary to keep the centre of gravity low, and it was not till much later that this was generally recognised. In addition his engines were fitted with his patent expansion gear (known as the horse leg motion) which was the earliest to be used on locomotives, and his reversing lever had a notched sector".
Horse-leg motion Patent No 7745 26 July 1838
Boiler Patent 7306 17 February 1837 with John Chanter
See: E. L. Ahrons, The British Steam Railway Locomotive
Holcroft, H., The Armstrongs of the Great Western. 1953 pp 28-9
Bradley, D. L., Locos of the LBSCR Part 1 (RCTS) 1969