Four Union Pacific diesel locomotives, including two cabless "booster" units, leading an eastward freight train toward the summit of Sherman Hill in Wyoming on September 18, 1977. The booster units are eight-axle DD35Bs built by General Motors' Electro-Motive Division. Cabless booster units were common among first generation diesels of the 1940s and 1950s, but with the introduction of more powerful second generation locomotives in the 1960s, most railroads preferred the flexibility of having cabs in all of their locomotives. Only Union Pacific and Southern Pacific ordered second generation booster units (see record 2063
for another model). Both railroads operated long, heavy freight trains across great distances in the west using several locomotives, and Union Pacific, in particular, regularly experimented with exceptionally large steam and diesel locomotives. These DD35Bs, built in 1964, were retired in 1979, just two years after this photograph was taken.