The ALCO RS-3m is a diesel-electric locomotive rebuilt from an ALCO RS-3 road switcher. These 98 locomotives were rebuilt to replace their original ALCO prime mover with the more reliable EMD 567B engine and fan assemblies taken from retired E8s. Many of these rebuilds were performed by the ex NYC DeWitt shop with 56 completed at the ex PRR Juniata shop. The RS3m rebuild program started in 1972 and continued until 1978 under Conrail.

The ALCO RS-3 is a 1,600 hp (1.2 MW), B-B diesel-electric locomotive manufactured from May 1950 to August 1956 by American Locomotive Company (ALCO) and its subsidiary Montreal Locomotive Works (MLW). A total of 1,418 were produced: 1,265 for American railroads, 98 for Canadian railroads, 48 for Brazilian railroads, and seven for Mexican railroads.