The original Yorkshire Engine Company, established in Sheffield in 1865, had a moderately successful existence supplying steam locos to local industry and overseas. It also made mining machinery, had a dabble at motor car manufacture and is strangely well-known for locos supplied to the Romney Hythe and Ravenglass & Eskdale railways. A lot of YEC locos were operating in steelworks, and it was absorbed in 1945 by the United Steel Company. This gave it a "captive market" and the opportunity to refine its designs by close liaison with operators, as a result the "Janus" twin-engined 0-6-0DE was a very well regarded industrial loco still in service today, fifty years after it first appeared.
Apart from "YORKSHIRE", YEC also badged their locos with the above, sometimes with the slogan "A good pull from the start".
Its works were based at Meadowhall, by today's Tinsley (M1) viaduct, and indeed, the clearance of the lower deck of the viaduct is owed in part to YEC indicating the minimum heights needed to facilitate road-borne despatches. Although it had just produced 74ton "Olympus" Bo-Bo's at 5'6" gauge for Durgapur steelworks (an Indian offshoot of United Steels) the overall prospects for loco manufacture worried the United Steel board and they wound up the business in 1965, with all designs etc, being acquired by Rolls-Royce.
There is also reference on this website to "another" Yorkshire Engine Co, which Andrew's father ran from 1989 to 2001.