DEEX's high-horsepower fleet consisted of 12 SD40s #001-006 delivered during 1970 and #012- 017 which followed two years later. Interestingly, the final group of Edison 40s were built in 1972, well after EMD had been delivering SD40-2s. During that same year, General Electric delivered ten U30Cs (#007-011 and 018-022) to the Detroit-based electric company. All 22 units, at the time, sported EMD's popular blue & silver demonstrator color scheme.

The EMD/GE fleet served the utility company for more than 20 years. On more than one occasion, the private sector energy company leased three or four locomotives to power-starved Rock Island, during 1972/73 and 1984. When retirement arrived, the entire fleet was "scattered to the four winds." The second-hand locomotive market gobbled up several units, with Helm Leasing (HLCX), Guilford, and Lake Superior & Ishpeming acquiring multiple units. The LS&I, purchased three of the six-axle GEs to serve as part sources for the railroad's existing U30Cs. Shortly afterwards however, the recent arrivals were quickly leased to other Upper Peninsula railways, which were experiencing a power shortage. Lastly, Helm's ex-DEEX SD40s worked in lease service on numerous railroads for many years. The last occurence took place during May of 2009 on short-line Nebraska Kansas and Colorado.

Reportedly, the last surviving DEEX U30C was acquired by a private owner and stored in Tennessee at the SARM.