Housatonic Railroad 9935 was originally built for the Delaware, Lackawanna & Western as a model RS-3 diesel-electric locomotive by the American Locomotive Co. (ALCO) in Schenectady, NY. Built in April 1951, it was equipped with a Model 244, twelve-cylinder diesel engine generating 1,500 horsepower. Built as DL&W 910, it became Erie Lackawanna 1048 in 1960. When Conrail took over the operations of EL in 1976, it became 5264. Less than six months after takeover, Conrail retired the unit in October 1976.
Conrail tried to extend the life of their retired Alco switchers by rebuilding them with EMD prime movers from recently retired E8’s. Conrail's Altoona, PA shops rebuilt 5264 in March 1979, replacing the Alco prime mover with an EMD 12-567-B diesel engine from a scrapped E-8. It was designated a model RS-3m ("m" for modified) and assigned number 9935. A few years later, Conrail discontinued the rebuild program and retired the RS-3m’s.
The Housatonic Railroad purchased number 9935 in 1986, and used it extensively during their startup operations. Sidelined for many years, the unit was moved to Lenox where Berkshire Scenic Railway Museum volunteers reactivated the unit in 2005. It has been on loan to the BSRM ever since, and is occasionally recalled to freight service or work train service by the Housatonic Railroad.