NBL had entered into an arrangement with the German company MAN AG in the early 1950s to market MAN's engine designs in the UK: NBL was anxious to enter the diesel locomotive market, especially once it became apparent that British Railways would be seeking large quantities of such locomotives when the "Modernisation Plan" was announced.The first results of this collaboration were the D600-D604 locomotives which failed to take advantage of the weight-saving potential of light alloy stressed-skin construction allied to hydraulic transmissions.The prime mechanical components of these were two MAN L12V18/21B diesel engines, each rated at 1,100 hp (820 kW) at 1530 rpm and coupled to a Voith LT306r hydraulic transmission; each engine/transmission combination drove one bogie.Unlike the Mekydro four-speed transmissions in the Swindon-built locomotives, the Voith was only a three-speed design but was chosen because it kept compatibility with D600-4 and because NBL already had a licence to manufacture it.[7] The NBL-built D800s were withdrawn before their Class 42 sisters, themselves doomed to a short life because of the decision to standardise on diesel-electric transmission for mainline locomotives.
D836 'Powerful' hauling a Paddington-Bristol express in 1962