In the mid-1980's Santa Fe decided to upgrade and extend the lives of their original SD39 fleet. Beginning in 1985 these units were re-manufactured by the San Bernardino CA Shops and reclassified as SD39u's. They were renumbered into the 1556-1575 number class and were designated as heavy switchers. The upgrades included updated wiring, new controls for hump service, and completely rebuilt engines. They were assigned to Barstow CA and Argentine KS yards, but could be found roaming the system. They were mated to rebuilt six-axle slugs for hump service. Our models include the specific details such as, relocated nose headlight, tall cab-mounted horn, antenna with stand, and front/rear slug MU boxes. Since Santa Fe was known for changing out air conditioners, we included both Prime and Vapor versions.