GE seems to change their model designation alot(probably marketing...). The AC4400 or AC4400CW is the AC version of the basic 16 cylinder GE locomotive. The ES44AC is the new 'GEVO' model. It has a new 12 cylinder engine with a larger 'split cooling' radiator section. They are very different if you compare them 'side by side'.
EMD's 'enviromental' offering is the SD70ACe - and again, this locomotive is externally different than the SD70MAC.
Both builders had to design new locomotives to meet the EPA emmission requirements for new construction locomotives.
Jim
