Union Pacific Slug S4 began life as cabless EMD UP GP9B No. 316B that was constructed in September 1957. After the locomotive was retired in August 1975, it was rebuilt into Slug S4 for hump yard switching. Basically, a slug is a locomotive frame with only traction motors and added ballast weight, and is semi-permanently coupled to a locomotive. In this case S4 is mated to SD24 No. 446 for switching Bailey Yard at North Platte, Nebraska, as seen in June 1981. It still looks much like a GP9B externally, but has added large outside sand boxes over the trucks. A total of 165 GP9Bs were built by EMD, with UP buying 125 units and Pennsylvania Railroad acquiring the balance.
