The Baldwin Class 10-12-D is a narrow gauge 4-6-0 pannier tank steam locomotives built by the Baldwin Locomotive Works in the USA for the British War Department Light Railways for service in France during World War I. They were built in 1916-1917 to 600 mm (1 ft 11 5⁄8 in) gauge.
The British War Office decided to adopt the type as its principal military steam locomotive, and Baldwin started production in 1916, building 495 locomotives between October 1916 and April 1917. Some of the class found their way to other theatres of war than Europe and nine were lost at sea. After the war many of these locomotives were sold and went on to work in France, Britain and India.
British narrow-gauge railways which used them included Welsh Highland Railway, Glyn Valley Tramway, Snailbeach District Railways and Ashover Light Railway. Four Baldwin Class 10-12-D locomotives have been preserved in the UK.