The ES (Evolution Series) of locomotives were designed to meet the U.S. EPA's Tier 2 locomotive emissions standards that took effect in 2005. With a newly designed "GEVO-12" 12 cylinder engine, these AC traction locomotives have the same 4,400 horsepower as the 16 cylinder AC4400CW but produce fewer emissions and have greater fuel economy. Possibly their most identifiable trait is their “oversized” radiator, courtesy of a specially designed air-to-air dual fan heat exchanger.

The Canadian Pacific was the first North American railroad to put AC traction locomotives into use, and today has the highest percentage of AC vs. DC locomotives in service out of all North America’s class 1 railroads.