In early 1973, Santa Fe instituted a rebuilding program for their large fleet of SD-24s; the end result of this program was the SD-26. Fairly extensive modifications were made to the locomotives, making them visually distinct from stock SD-24s. The addition of an inertial air filtering system created a ?hump? directly behind the cab. This, in turn, caused the 4 roof-top air tanks to be separated and relocated further back along the edges of the roof. A ?second generation? EMD-style traction motor blower bulge was included on the left side of most rebuilt locomotives. Other spotting features included the addition of a cab-top beacon and air conditioner on most units, as well as the relocation of the bell to the roof. Most SD-26s were retired by Santa Fe in 1985 and 35 of these units were sold to Guilford Transportation Industries in 1986. Of this group, at least 3 were still in service for Guilford as of January 2004, over 44 years after they were built.