The MLW RS18 was the successor to the 1,600hp 244-powered RS10 which proved to be somewhat unreliable. There were no such issues with the 1,800hp RS18, with its 12-cylinder model 251B diesel engine, and it is widely considered to be MLWs most successful product from both an operational and a mechanical point of view.
Based on the Alco RS11, the Canada-specific RS18 (which MLW designated DL-718) was alike in most aspects externally and internally except for the notchless hood ends.
Montreal Locomotive Works constructed 351 RS18s between December 1956 and June 1968 for eight railways including Canadian National (225), Canadian Pacific (72), Pacific Great Eastern (29), and 25 for smaller railways.
The RS18 was available in both "standard" and "lightweight" versions, which usually, but not always, rode on standard 9'-4" wheelbase AAR type B trucks or special lightweight trucks respectively. CN acquired examples of both (131 standard versus 94 lightweight units). Canadian National's fleet of 225 units, numbered 3615-3893, was delivered between December 1956 and August 1960, designated MR-18-a to MR-18-g. This included 30 units with 80-mph gearing and steam lines for passenger service. The last was retired in 1993 after nearly four decades of service.
Canadian Pacific's units, numbered 8729-8800, were assigned classes DRS-18a and DRS-18b and were delivered between March 1957 and July 1958. All were equipped with a steam trainline so that they could be partnered with a steam generator equipped unit. During 1989, CP began a major rebuild program at Angus Shops, Montreal, to extend their useful lifespan another 10-15 years. Eventually all 69 remaining locomotives were upgraded, gaining the designation RS18u.
Both the CN and CP MLW RS18 fleets mostly worked across Ontario, Quebec and the Maritimes, although CP's units occasionally worked west of Winnipeg.