 The locomotives are fitted with Alco 12-251B engines, developing 1343 kW.They were built in two batches, the first 60 between July 1957 and April 1961 and the second 40 between October 1965 and December 1967.The last 40 had Associated Electrical Industries as opposed to General Electric generators and traction motors as well as illuminated segregated number boxes at the No.2 end.