CN cut down and rebuilt both MLW S-3 and GM GP9 locomotives into booster units (slugs). They also bought some new units from GM Diesel Division in London Ontario. The power for the S-3 slugs was provided by MLW S-13 or GM GP9RM locomotives. The other slugs were powered by GP9RMs or GP38-2ms. Some slugs were designated Hump units and others were designated yard units. Even numbered S-13 & GP38-2m locomotives were lead units and others were trailing units.
The slugs are seen in action in Nova Scotia, New Brunswick, Quebec, Alberta and British Columbia.