- In 1955, Union Pacific ordered a new turbine-electric, the world's most powerful locomotive. Each would be two units plus a fuel tender, rated at 8,500 horsepower (6,300 kW).
- The locomotive weighed about 610 tons with a full tender. Continuous tractive effort was 146,000 lb (66,000 kg) with the 65 miles per hour (105 km/h) 74:18 gearing; in 1961 tonnage ratings were 6740 on the 0.82% climb west from Cheyenne and 5180 on the 1.14% east from Ogden.
- The turbines in these units are the most powerful prime movers ever installed in any North American locomotive. With 8,500 hp (6,300 kW) from a single prime mover, these engines set a record that still stands
- Unlike the earlier turbines, the 8,500 hp (6,300 kW) turbines came with 24,000 US gal (91,000 l) fuel tenders, in addition to the 2,500 US gal (9,500 l) of diesel fuel in the locomotive tank. They had Leslie S-5T-RF air horns on the cab roof (later moved to the mid radiator section of the A unit, in response to ice build-up in the bells).
- The third generation turbines were all retired by 1970.