Converted from retired MLW S-3 switchers, engines and cab removed, new carbody constructed. Slugs (known as boosters on CNR) have no engine and must be electrically connected to a locomotive (often referred to as a “mother” unit) that supplies electrical power to the booster’s traction motors. The advantage of a slug is that at low speeds the mother unit is producing more electrical power than its own traction motors can use. This excess power is used to power the booster’s traction motors, producing additional tractive effort without using additional fuel. The boosters were originally numbered in the B1 – B15 number series, all were renumbered. Some were assigned to the 300, 160, 265 and 700 series over the course of their service lives.
Trucks: B-B AAR Type A
No engine
Gear ratio 75:16
Maximum speed 40 mph
Operating weight 247,000 lbs.
Tractive effort 36,000 lbs.
Length 44′-5 3/4″
| Road no. | Class | Former road nos. | Subsequent road no. | Retired | Notes / disposition |
| 160 | Mh-00a | 302, B-2 | Mar 18 91 | To Sidbec-Feruni for scrap Apr 93 | |
| 161 | Mh-00a | 302, B-3 | Oct 20 95 | To Sidbec-Feruni for scrap Feb 2 96 | |
| 162 | Mh-00a | 303, B-4 | Dec 8 88 | ||
| 163 | Mh-00a | 306, B-7 | Oct 20 95 | To Tri-Province for scrap Feb 2 96 | |
| 164 | Mh-00a | 310, B-11 | Nov 20 96 | To Southern Marine 1998 | |
| 165 | Mh-00a | 304, B-5 | 262 | ||
| 166 | Mh-00a | 307, B-8 | July 30 92 | Sold to Sidbec-Feruni for scrap Apr 93 | |
| 167 | Mh-00a | 308, B-9 | 265 | ||
| 168 | Mh-00a | 309, B-10 | Jan 24 97 | To Port Colborne Harbour Railway 1999, scrapped 2012 |