General Electric U28C phase guide and builder's list
The U28C was a bridge model between the 2500 hp U25C and the highly successful U30C. The 2800 hp rating was a bit of a disadvantage compared to Alco's C630 and EMD's SD40, and the model remained in production for just over a year. Within months of the start of production, GE completely re-engineered the Universal Series with a new frame, car body, equipment layout, and in the case of the six-axle units, the new FB-3 floating bolster truck. Here's the link to the .pdf of the phase study and builder's list.