This is Hawthorn & Leslie 0-4-0ST (ST meaning 'Saddle Tank', meaning the water tank sits like a saddle around the top of the boiler) No 2, built in 1911 at their Forth Bank Works at Newcastle-upon-Tyne for Keighley Corporation Gas Department in the West Riding of Yorkshire where it received the number 2 in their fleet. In 1940 it went to the Ministry of Supply to work at the Royal Ordnance Factory at Dumfries, Scotland. It then worked at various munitions factories, Grangeston in Girvan, Ayrshire from 1949-1954, then Powfoot, Annan, Dumfriesshire from 1949-1976 (where it was again numbered No 2), before entering preservation in 1976 when it arrived at the Tanfield Railway.
The locomotive does not have a rear to the upper half of the cab meaning it is possible to view the goings-on on the footplate when in use. It is seen here in the later stages of an overhaul, later on the day this was took it was moved outside in preparation for it's overhauled boiler, seen undergoing a steam test here; http://www.tanfield-railway.blogspot.co.uk/2012/07/hl-no2-steam-test.html
Hopefully No 2 will be back in service soon and i'll have a photograph of the entire locomotive in action