Built by General Electric's Transportation Systems division in response to the introduction of tighter emission policies that came into effect in 2005, the ES44AC and DC locomotives replace the AC4400CW series. The upgrades result in more power and less emissions from the smaller GEVO 12-cylinder engine than its 16-cylinder predecessor.

Consisting of a twin six axle or Co-Co wheel arrangement, more than 2000 of these locomotives have been ordered by nearly all the major US and Canadian railroad companies, 506 of which are allocated to Union Pacific Railroads alone.

The Evolution Series locomotives are very similar in appearance to the Dash 9, with both AC and DC versions featuring the large cabinet behind the crew compartment on the left side, housing the Traction Inverters for the AC models. The radiators on these locomotives are longer than previous models, extending forwards towards the exhaust vents. Also present is a raised hump housing heat exchangers related to the reduced emissions.

These newer improved models have further enhanced the reputation of GE Transportation Systems to produce powerful heavy haulage machines for many freight carrier applications.

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