12343 darjeeling mail hauled by

A classic!

The answer is: "BB" - B Class steam locomotive.

The Darjeeling Himalayan Railway, also known as the "Toy Train", is a narrow-gauge railway that runs from Siliguri to Darjeeling in West Bengal, India. The railway was built in the 19th century and is a UNESCO World Heritage Site. The B Class steam locomotives, also known as "B-B" or "B-B-B" locomotives, were used to haul the Darjeeling Mail, a passenger train that ran between Siliguri and Darjeeling.