The ES44C4 (Evolution Series, 4400 HP, C to denote 3 axles per truck, 4 traction motors) was introduced in 2009. While similar to the ES44AC, the ES44C4 has two traction motors per truck, instead of the conventional three such as on the ES44AC. No ES44C4s with DC traction were built. The center axle of each truck is unpowered, giving an A1A-A1A wheel arrangement. BNSF Railway is the launch customer for this model, ordering an initial batch of 25 units numbered 6600-6624. As of 2020, BNSF Railway currently rosters over 1200 ES44C4 units The ET44AC (Evolution Series Tier 4, 4400 HP, AC traction) replaces the ES44AC model. These locomotives have been ordered by most of the Class I railroads in North America, including Union Pacific, CSX Transportation, Norfolk Southern and Canadian National Railway. BNSF Railway will be receiving similar ET44C4 locomotives, which have no traction motor on the middle axle of each truck. CSX designates their ET44ACs as ET44AH, due to their locomotives being Heavy-ballasted.