Mechanically identical to the Dash 8-40CW, the Dash 8-40CM was constructed only for Canadian railways, with a production total of 84 units. It is distinguished from the Dash 8-40CW by the addition of a full-width cowl body and the use of a Canada-specific nose and windshield configuration. The trucks also differ, all units built ride on Dofasco 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) V167FDL diesel engine driving a GE GMG187 main alternator. The power generated by the main alternator drives six GE 752AG or 752AH direct currenttraction 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).
Depending on customer options, the Dash 8-40CM carries approximately 5,000 US gallons (19,000 L) of diesel fuel, 410 US gallons (1,600 L) of lubricating oil, and 380 US gallons (1,400 L) of coolant. Like most North American diesel locomotives, the Dash 8-40CM uses normal water for cooling. The Dash 8-40CM has a maximum tractive effort of 108,600 lbf (483,000 N) at 11 mph (18 km/h) with the 83:20 gearing. Overall dimensions for the Dash 8-40CM are 70 ft 8 in (21.54 m) in Length, 15 ft 4 in (4.67 m) in height and 10 ft 2 in (3.10 m) in width.