275 mm length of a very light Vignoles rail (20 lb to the linear yard). This was the lightest weight commonly rolled and was used only for the lightest traffic. The fishing angles do not seem quite equal, and that again suggests crude track. The source is probably a colliery railway as these were present in the area around Gatehead (OSGR NS392365) where it was found. Many of these colliery railways took coal to the Kilmarnock and Troon for onward transport.
A plan of the area in which the rail was found is found on the northern edge of the OS 6inch map XXII for Ayrshire (surveyed 1857 and published 1860) in the vicinity of Gatehead. See link: http://maps.nls.uk/os/6inch/view/?sid=74425809.