The last steam engines operated by Union Pacific represented the pinnacle of railroad steam power development in the 1940s. Steam operations ended on Union Pacific in the late 1950s, though UP has retained two of those "modern" steam locomotives for historic and public relations purposes.
The introduction of alternating current technology (AC) to modern diesel-electric locomotives is expected to be as significant as the conversion from steam to diesel for railroad economics.
Union Pacific has retained four historically significant diesel-electric locomotives for special operations. The company also has donated scores of early model diesels to various museums across the United States.
Today Union Pacific operates the largest diesel-electric fleet in the United States.