This is a detail of the safety valve on the De Winton vertical marine boiler fitted to the locomotive 'Taffy'. In the early years of boiler development they had various automatic spring loaded valves which vented excess steam when the boiler pressure began to rise above it's safe working pressure. It was not unknown for locomotive drivers to load their safety valves in order to obtain increased boiler pressure so that they could make up lost time. Such practice was dangerous and a number of boiler explosions were attributed to it. In 1856 the eminent locomotive engineer John Ramsbottom designed a tamper free duplex safety valve. The valves in this picture appear to be based on this design. This type of safety valve prevented any loading which would result in an increase of boiler pressure, but did allow pressure to be released by means of a lever which had contact with both valves. For more information about John Ramsbottom, see:
The picture was taken at the opening celebrations for the new workshop and motive power depot on the Golden Valley Light Railway at Swanick Junction in Derbyshire.