The ET44AC (Evolution Series Tier 4, 4400 HP, AC traction) replaces the ES44AC model. These locomotives have been ordered by most of the Class I railroads in North America, including Union Pacific, CSX Transportation, Norfolk Southern, KCS, and Canadian National Railway. BNSF Railway will be receiving similar ET44C4 locomotives, which have no traction motor on the middle axle of each truck.

In the summer of 2012, GE unveiled its first Tier 4 emissions-compliant locomotive, now officially named the ET44AC. The prototype demonstrators, as well as current production units, are visually quite different from previous models. First, the "ET" is 74 feet 6 inches (22.71 m) long - an increase of 16 inches (0.41 m) from the 73-foot-2-inch (22.30 m) length of all previous GE 6-axle models beginning with the "DASH 9" series in 1993. While the front third of the locomotive, including the control cab, is basically unchanged, the rear two-thirds is noticeably different. On Tier 4 demonstrator units the entire midsection above the prime mover is raised to accommodate an advanced exhaust treatment system; it is seen as a "hump" that rises above the rest of the locomotive considerably. Production units have this hump only over the exhaust manifold directly ahead of the radiator, rather than encompassing the entire midsection. In addition, the radiator section is much larger than that of earlier "ES" units, featuring a two-fan split-cooling system located inside a very large, angular compartment. Grilles under the radiator section have been smoothed out, and there are two extra air intakes on the rear of the locomotive hood. ET44ACs are all equipped with Graham-White electronic bells (E-bells). Tier 4 demonstrator units are equipped with two airhorns: a three-chime horn – facing forward – located behind the cab and a rear-facing two-chime horn protruding from the back of the radiator. Production units feature the now-standard Nathan-Airchime model K5HL-R2 "Evolution" air horn mounted on the hood slightly forward of the placement location used on previous models.A DC variant of the ET44AC has not been cataloged by General Electric,as railroads prefer AC traction units.