Part of GE's "Dash 9" series of locomotives, the Dash 9-40CW shares its frame, HiAd trucks, 16-cylinder 7FDL engine, and 752AH-31 traction motors with the GE Dash 9-44CW. But while the more common 9-44CW offers 4,400 horsepower (3,300 kW), software in the 9-40CW's engine-governing unit restricts its power output to 4,000 horsepower (3,000 kW), although the engineer can override this restriction when desired. The Dash 9-40CW was basically an extension of the previous order for the standard cabGE Dash 9-40C, built under the same premise that a lower power rating would prolong the life of the engine and use less fuel.
In 2005, GE replaced the C40-9W in its locomotive catalog with the Evolution Series model, the ES44DC. Like the Dash 9-40CWs, Norfolk Southern's ES40DC locomotives are rated at 4,000 hp (3,000 kW) to conserve fuel. The change in output is accomplished electronically; mechanically, the two models are essentially the same.
In late 2013, NS started to upgrade their C40-9Ws into C44-9Ws by increasing the wattage from 3,000 to 3,281 kW (4,023 to 4,400 hp), thus increasing the horsepower rating to 4,400 hp (3,281 kW). The units are being upgraded as they go in for their 184-day maintenance cycle.
As of 2015, all units have been upgraded to 4400 and are classified as D9-44CW. 
As of 2017, there are plans to rebuild some earlier (unknown which numbers) to AC44C6CF. The AC44C6CF in the same as the AC44C6M, but the 'CF' units will use a TMV control system and Interstate electronic fuel injection. They will be renumbered in the 85xx series.