The U28C was built in two distinct Phases, Phase I utilized the U25C carbody, the significant spotting feature being the step-up in the running board at the location of the radiators at the rear of the unit, and drop equalizer trucks. The Phase II introduced a General Steel Castings truck similar in design to EMD's Flexicoil truck, and a truck of similar design referred to as the Adirondack truck. The radiators of the Phase II had a bulge on the side just above the intakes, and a walkway over the center on the roof. When Penn-Central came into existence February 1, 1968, the U28C's retained their PRR road numbers. On April 1, 1976, Conrail took over the failed PC and several other eastern railroads. To make room for SD40-2's, CR re-numbered the U28C's into the 6820-6834 series, and they lasted until retired in 1983.