The SD50S ("short frame") were prototype units built in December 1980. They were shorter than production locomotives by approximately 2 feet (0.61 m). All of these prototypes were sold to the Norfolk & Western and passed to the Norfolk Southern; they were withdrawn in the early 2000s as non-standard. Two were rebuilt in 2008 as "SD40E" models by Norfolk Southern's Juniata Shops, along with several standard length SD50's.
The designation also applies to five locomotives built by EMD Australian licensee Clyde Engineering for Hamersley Iron. These were also shorter than production SD50s, and were equipped with a special double cab roof for insulation against the hot Australian desert sun. When withdrawn from Hamersley Iron service, they were sold to National Railway Equipment Company and exported to the U.S. in February 1999 and used in national lease service. They were subsequently sold to the Utah Railway in June 2001, where they remain.
Read more about this topic: EMD SD50