[citation needed] Some SW900s were built with the generators from traded in EMC Winton-engined switchers and were classified as SW900M by EMD.

[citation needed] In the early 1960s, the Reading Company sent 14 of their Baldwin VO 1000 model switchers to EMD to have them rebuilt to SW900 specifications.

The Reading units retained the Baldwin switcher carbody and were rated at 1000 horsepower by EMD.

[citation needed] In 1974, the Southern Pacific Transportation Company had decided to rebuild all ten of their EMD SW900 diesel locomotives at their own Houston Shops.

The rebuilds were done specifically to extend the life of the locomotives and to increase their power and reliability.