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The Middleton (Colliery) Railway is the world's oldest working railway.
The Middleton Steam Railway built for the Middleton Colliery was the first railway to be authorised by Act of Parliament in 1758. It was built to a gauge of 4 ft 1 in to carry coal from the pits owned by Charles John Brandling to Leeds. Not all the land belonged to Brandling and the Act gave him power to obtain wayleave. Otherwise the line was privately financed and operated, initially as a wagonway using horse-drawn vehicles.
Around 1807 the wooden tracks began to be replaced with edge rails.
In 1812 the Middleton Steam Railway became the first railway to commercially use steam locomotives. John Blenkinsop the colliery's viewer, or manager, decided that an engine light enough not to break the track would not have sufficient adhesion. Accordingly he relaid the track on one side with a toothed rail, which he patented in 1811, and approached Matthew Murray of Fenton, Murray and Wood to design a locomotive with a pinion which would mesh with it.
Murray's design was based on Richard Trevithick's Catch Me Who Can, adapted to use Blenkinsop's rack and pinion system, and was called the Salamanca. The loco was the first to use two cylinders. These drove the pinions through cranks which were at right angles, so that it would start wherever it came to rest.
1812 The line first ever to operate successfully, and with three more locomotives built later, remained in use for another twenty years.
1817, 28 February. A steam-impelled engine belonging to J. C. Brandling Esq and employed on the railway of his colliery blew up and killed the driver and scalded several children - Leeds Mercury . The locomotive was the Salamanca, and the driver had prevented the safety valves from working. 'Richard Jackson, the manager of Messrs. Fenton, Murray and Wood's Steam-Engine mannufactory, at whose works the boiler of the Salamanca was cast, stated that he examined the broken boiler:- it appeared to have been perfectly cast, of sound good metal, and to have been burst by negligence, in keeping the spring upon the safety-valve, at a time when the Engine was not in motion.' 
In 1881 the railway was converted to 4 ft 8.5 in standard gauge.
1816. The Times newspaper reports on the journey of Lieut-Colonel Fischer of Schafhausen. 
'At Leeds he notices the waggons driven by steam, which conveyed coal along an iron railway, three miles in length, to the town. "I went to meet this train" says he "two miles off; and when I came up to it, the man guided the whole desired me to mount the waggon of the machine, which was provided with seats; and the usual rate of its motion being such, that a man walking rather fast can scarecly keep up with it; in order to satisfy my curiosity he increased its rapidity to that of a trotting horse, by a stronger application of steam to put in motion the stampers which moved at the rate of 80 strokes in a minute. I was very glad when he made it move more slowly, as I was afraid of an explosion, because the steam hissed as if our vehicle was drawn along by half-a-dozen of broken-winded horses. For the rest, I rejoiced to enter Leeds seated in this triumphal car of human ingenuity (or so I would call it) where the elements confined in so small a compass themselves compel 23 waggons laden with 60 cwt of coal each".
See  Middleton Railway Web site