The ET44AC is a derivative of the ES44AC, which complies with the EPA Tier 4 emissions standard. Visual distinguishing features are the raised fairing on the exhaust system and larger coolers at the rear. In the meantime, no variant with a DC traction motors is offered anymore. The more than 1,000 copies so far are spread mostly between the BNSF, Canadian National, CSX and Union Pacific. The BNSF locomotives are ET44C4, which, like the ES44C4, only have four traction motors and are suitable for faster, not quite as heavy freight trains. 