In the 1960s, railroads were looking to replace aging EMD E-units and Alco PAs for passenger service. EMD responded by adding a steam generator compartment to the long hood end of their SD45. This required a longer frame and resulted in the creation of the SDP45. Three railroads originally purchased SDP45s: Southern Pacific, Great Northern, and Erie Lackawanna. Over the decades, the SDP45s were then acquired by other railroads through mergers, leasing companies, and rebuild programs.