This locomotive was built in 1944, to a Hunslet Engine Company design, by Robert Stephenson & Hawthorn for the Ministry of Defence.
See LNER class J94 for details.
Initially the locomotive was sent to the Ashington Coal Company by the War Department to help handle the demand for coal.
When the war came to a close it spent time at Horton Grange (Northumberland) opencast disposal point before being sent to Swalwell Disposal Point in what was then County Durham (now Tyne & Wear). At the time, the opencast disposal point was worked by Mechanical Navvies Limited which gave rise to the name the locomotive carries
The locomotive remained there until 1974 when it moved to the East Somerset Railway. The opencast site it had worked on became the Metro Centre at Gateshead in the 1980s.
It was taken into preservation by 1974 to the East Somerset Railway, where shortly after arrival it was liveried in BR black with the BR number 68005 although it never ran in BR service.
By 1995 the locomotive was based at the Bolton and Embsay Railway, still in BR black and where it was nicknamed ‘The Mog’. Although overhauled in 2004, a burst boiler tube two years later resulted in the locomotive being taken out of service and stored.
In August 2009 the locomotive was purchased by a Pontypool & Blaenavon Railway member and moved to the Flour Mill for repair the following month.
The locomotive arrived at the Pontypool & Blaenavon Railway in April 2010 and it then worked most of the steam hauled trains on the line.
The locomotive is still based on the Pontypool & Blaenavon Railway and is operational.