It is distinguished from the Dash 8-40CW by the addition of a full-width "Draper Taper" style cowl body, of which they are the last locomotives built with this feature, and the use of a Canadian-specific nose and windshield configuration.

The trucks also differ; all units built ride on Dofasco Hi-Ad trucks identical to locomotives built by Montreal Locomotive Works, Bombardier, and Morrison-Knudsen.

The Dash 8-40CM is powered by a 4,000-horsepower (3,000 kW) 7FDL-16 diesel engine driving a GE GMG187 main alternator.

The power generated by the main alternator drives six GE 752AG or 752AH direct current traction motors, each with a gear ratio of 83:20 and connected to 40-inch (1,000 mm) wheels which allow the Dash 8-40CM a maximum speed of 70 mph (110 km/h).

Like most North American diesel locomotives, the Dash 8-40CM uses normal water for cooling.