Designed for use in Amtrak's inner-city lines, the F40PH series of locomotives was ordered as a replacement for the EMD SDP40F type, Amtrak's first series of locomotives built new for them since its inception in 1971. To support passenger service, the F40PH is equipped with a secondary electrical generator referred to as the head end generator (or HEP, for Head End Power). The need for constant constant frequency power means that these HEP units are constantly running at 900rpm, resulting in the nickname "Screamers" for the units due to the sound.