 All 10 examples of this locomotive model were built for Milwaukee Road, who wanted a lightweight road-switcher to replace their fleet of 1947-built ALCO RSC-2s.The unit was built on a short frame with C-C export trucks, barely tipping the scales at and managing a light-footed axle-loading of just per axle.The EMD SDL39 is practically an experimental, lightweight version of the SD39, but uses a GP38's body and frame with the SD39's engine.