A GE ES44C4, BNSF 4261, sits in a consist of quiet motive power in the Pueblo Railyard. The biggest difference between the ES44C4 and the ES44AC is that the middle axle of each truck on the locomotive is unpowered.
The BNSF logo on the side of the locomotive:
The front truck of the engine. I think the cylinders on the truck have something to do with transferring weight from the powered axles to the unpowered middle axle, but I don’t know exactly how it works. The model designation decal above the truck helps to confirm the unit’s type. A trend I’ve noticed, but haven’t confirmed, is that the model designation for BNSF’s ES44AC is green rather than the yellow you see on the ES44C4s. I wouldn’t call that a rule, per se; for all I know the different color paint on the model designation could be as simple as the result of where the engine was built, rather than a conscious effort by BNSF to distinguish models.
(Edit: Since writing this, I’ve seen the designation in the green paint, so if there’s any significance to it at all, it’s probably where the engine was manufactured. If you know, please tell me in the comments. Thanks!)