15 units, built by CN 1964 – 1965
B-B
Operating weight – 247,000 lbs.
Tractive effort – 36,000 lbs.
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. Nos. B-1 to B-15 were used in hump yard service, initially in pairs between a pair of MLW S-13 switchers.
| Road no. | Class | Rebuilt from | Rebuilt | Renumbered to | Renumbering date |
| B-1 | MH-00a | 8477 | Jun 3 64 | 300 | c. 1969 |
| B-2 | MH-00a | 8452 | Jun 3 64 | 301 | c. 1969 |
| B-3 | MH-00a | 8462 | Dec 2 64 | 302 | c. 1969 |
| B-4 | MH-00a | 8465 | Jul 22 64 | 303 | c. 1969 |
| B-5 | MH-00a | 8487 | Jan 9 65 | 304 | c. 1969 |
| B-6 | MH-00a | 8491 | Jan 24 65 | 305 | c. 1969 |
| B-7 | MH-00a | 8488 | Jan 9 65 | 306 | c. 1969 |
| B-8 | MH-00a | 8489 | Dec 2 64 | 307 | c. 1969 |
| B-9 | MH-00a | 8479 | Dec 15 64 | 308 | c. 1969 |
| B-10 | MH-00a | 8495 | Feb 25 65 | 309 | c. 1969 |
| B-11 | MH-00a | 8466 | Feb 14 65 | 310 | c. 1969 |
| B-12 | MH-00a | 8494 | Mar 13 65 | 311 | c. 1969 |
| B-13 | MH-00a | 8464 | Mar 3 65 | 312 | c. 1969 |
| B-14 | MH-00a | 8450 | Sep 23 66 | 313 | c. 1969 |
| B-15 | MH-00a | 8451 | Dec 14 66 | 314 | c. 1969 |