Amongst the foremost miniature railway engineers of the 20th century, remembered as a master of engineering design.

Greenly is perhaps best remembered for his miniature locomotive designs. He worked closely with many engineering companies, including Bassett-Lowke and its various engineering subsidiary companies. In 1909, along with Wenman Joseph Bassett-Lowke, Henry Greenly started and edited Model Railways and Locomotives Magazine.

He worked with Captain J E P Howey on the designs for the world-famous Romney, Hythe and Dymchurch Railway in Kent, England, and served as that railway's Chief Engineer. He was also involved with innovative locomotive design work at the nearby Saltwood Miniature Railway.