July 1973 is when Amtrak placed the first of their SDP-40F's in revenue service.
From A-Day until Spring 1972, Amtrak had leased "Warbonnet' F-Units, but even if those locomotives "looked sharp', their relaibility was questionable. Thereafter, In an effort to improve on-time performance as well as to minimize the possibility of a "broken down Amtrak train tying up the railroad", AT&SF 'released' for Amtrak service, steam generator equipped FP-45's. Although these locomotives when delivered during 1967 were liveried in 'Warbonnet', subsequent to A-Day they had been repainted to ATSF "freight Blue' livery.
At times, there would be one unit each of a non steam generator F-45 and one of an FP-45; Santa Fe passenger equipment had less requirement for steam, especially during summer, as the Hi-Leverl cars all had internal "hotel' power generator units.