 A requirement for a locomotive with a larger driving wheel diameter for higher speed train service on the more level parts of the railway led to the design of a new class.Initially a "Double Pacific" (4-6-2+2-6-4) was considered, but after examining the Sudan Railways 250 class (which were built in 1937), the RR settled on a design with the same wheel arrangement and same driver diameter () as the Sudanese Double Baltics.Initially four locomotives were ordered, and they were numbered 271–274.