General Electric introduced its Evolution series (“G.E. Evolution” ⇒ GEVO) of locomotives to comply with the ever-more-stringent EPA emissions standards. The initial models (e.g. ES44DC) were Tier 2 compliant, coming out around 2005. The latest versions, the ET44AC and C4, are Tier 4 compliant.
BNSF has purchased thousands of the emissions-friendly GEVO series, in several different models– both DC and AC traction, and the interesting ES44C4 and ET44C4 versions with only four powered axles in an A-1-A A-1-A arrangement.
For the railfan, there are a few spotting clues, but the most reliable is the model type painted under the cab. The front half of all types is nearly identical; the main differences are around the left-side inverter cabinets and especially the radiator structures. One subtle difference on the ET models is their increased length; they are 16″ longer. (But one glance at the radiators should identify them.)
This page has a sampling of roster photos of the various GEVO types that BNSF has owned.