| CN O-18-a | |
| Powertype: | Steam |
| Builder: | Grand Trunk Railway's Point St. Charles Shops |
| Builddate: | 1919-1921 |
| Totalproduction: | Fifty |
| Whytetype: | 0-6-0 |
| Uicclass: | C |
| Driverdiameter: | 513NaN3 |
| Locoweight: | 1740001NaN1 |
| Fueltype: | Coal |
| Boilerpressure: | 1752NaN2 |
| Cylindercount: | Two, outside |
| Cylindersize: | 22x |
| Valvegear: | Baker |
| Tractiveeffort: | 367032NaN2 |
| Operator: | Grand Trunk Railway » Canadian National Railways |
| Operatorclass: | GTR: F9 CN: O-18-a |
| Fleetnumbers: | GTR: 1749–1798, CN: 7424–7473 |
| Preservedunits: | Two preserved (7456 is on static display in Sidney, MI while 7470 is operational), remainder scrapped |
| Scrapdate: | 1954–1961 |
CN O-18-a is a class of 0-6-0 steam locomotives formerly owned by Canadian National Railways. They were switchers built by the Grand Trunk Railway's Point St. Charles Shops. Canadian National used the letter O to designate 0-6-0s.
The Grand Trunk Railway built 50 of these locomotives between 1919 and 1921 and designated them F9 Class. The GTR also had 25 more built in 1920 by the Lima Locomotive Works for use in the United States. In 1923, the Grand Trunk Railway was absorbed by the Canadian National Railways. CN classified the homebuilt locomotives O-18-a and the Lima-built F9s became GTW O-18-b class.
The O-18-a locomotives were numbered 1749–1798 by the Grand Trunk, and were renumbered as 7423–7473 by the Canadian National. They operated right up until dieselisation.
Upon retirement, 47 of the 50 locomotives were scrapped, but three were sold. CN 7439 (GTR 1764) was sold to International Harvester in 1958, but was scrapped only three years later in 1961. Canadian National 7456 (GTR 1781) and Canadian National 7470 (GTR 1795) were sold to Canada and Dominion Sugar (Chatham, Ontario) in 1959. These two survive to the present.
O-18-a number 7456 is at Montcalm Community College as a display locomotive, while number 7470 works at the Conway Scenic Railroad in North Conway, New Hampshire and is currently operational.