The ES44AC is the direct successor of the AC4400CW, which could not be produced domestically after 2004. Like EMD, GE had to essentially design a new locomotive to comply with EPA Tier 2 standards. Unlike EMD, GE had to give its new locomotive a new power source; the 7FDL16 prime mover was replaced with a more efficient prime mover, the GEVO12.

The ES44AC is selling very well, far surpassing EMD's output. The ES44AC has a few other variants. The ES44C4, introduced in 2009, has seen the most orders from BNSF. This type of ES44AC is a standard model, rather with the outer two axles of each truck being powered and the middle axle of each truck being a trailing axle. Another variant is the ES44AH, ordered by CSX, (classified as CW44AHs by the CSX, similar to their designation for the heavier AC4400CW predecessor) and as of late 2014, the Union Pacific (UP calls them C45AHs.) These units are weighted to 432,000 lbs compared to the usual 415,000 lbs. CSX's version utilizes GE's Radial Steering trucks. The Union Pacific has continued a tradition with their ES44AC locomotives as they have done with the AC4400CWs, in which they have been ordered with "Controlled Tractive Effort" for DPU service. Which brings the final designation to "C45ACCTE." The -45 in this case, does not indicate horsepower, rather it distinguishes these newer units from older AC4400CWs.

The ES44AC in Late 2011 was reworked to meet EPA Tier 3 standards which went into existence Jan 1st, 2012. There are no visible changes. However, in August 2012, GE released its Tier 4 ES44AC prototype, this locomotive looks very different from a Tier 2/3 ES44AC. Tier 4 regulations started on Jan 1st, 2015. This Tier is the final and most stringent of the standards. Several Tier 4 ES44AC's are testing on the UP, BNSF, and CN. Tier 4 ES44AC's retain most features of previous ES44AC's, but they have raised "humps" in the middle, as well as a larger, more sloped radiator in back with two cooling fans. These require more space in the car body for Exhaust Gas Re-circulation equipment. It has been confirmed that production units will not have the "humps" in their car-bodies. GE currently rosters around 30 Tier 4 ES44AC demo units (GECX #2015, 2020-2024, up until 2043).

In 2013, GE built an LNG (liquefied natural gas)-powered ES44AC, GECX #3000. This unit is equipped with a NextFuel LNG conversion kit and is currently used as one of two units in BNSF's GE LNG test set (the other unit is BNSF 5815, an older ES44AC also equipped with the NextFuel kit). 3000 and 5815 are separated by a BNSF LNG fuel tender  

2015: GE's locomotive plants are packed, with a backlog of two-three years. GE has Tier 4C units and production Tier 4 units under construction. The Tier 4 Credit units are the same as Tier 3 units under the hood, they have to be labeled as Tier 4C as part of the new regulations. GE earned credits by incorporating features into is EVO line that brought emissions under Tier 3 levels.  

2016: Orders for more Evolution Locomotives continue to roll in. BNSF, UP, CSX, CN and NS are accepting locomotives this year. The NS order is notable since it is their first order since Mid-2014. It is 47 ET44AC's (3600-3646) and 3 ES44AC's (8166-8168). All units from the 2016 order have been completed.  

2017: Orders have been slowing somewhat into this year however there are still plenty of new units rolling off the line. NS took delivery of 34 ET44AC's and 16 Tier 3-credit units. BNSF is still taking on ET44C4's and Tier 3-credit units. Two of the GE Demonstrators have been acquired by the Navajo Transitional Energy Co. #2026 and #2027 were rebuilt by GE and received a lower middle hood section like the phase I ET44AC's. They will replace the GE E60's currently used. (Canadian Pacific currently has no ET44ACs in their roster).