Canadian National class N-4steam locomotives were of 2-8-0wheel arrangement in the Whyte notation, or 1′D in UIC classification. These locomotives were built for the Grand Trunk Railway (GT) from 1906 until GT began purchasing class M freight locomotives in 1913. Initially classed D2 by the Grand Trunk, they were built as Richmond compound locomotives with 210 lbf/in2 (1.4 MPa) boilers feeding 22+1⁄2 inches (572 mm) and 35 inches (889 mm) by 32 inches (813 mm) cylinders.
The Grand Trunk started to rebuild and reclassify them; superheated compounds became class D3; those rebuilt with 23-by-32-inch (580 mm × 810 mm) simple expansion cylinders and Stephenson valve gear became classes D4, D5 and D6. The D4 was extinct by the 1923 takeover of the Grand Trunk by Canadian National Railway; CN reclassified the others as N-4-a and N-4-b respectively. Some locomotives received Walschaerts valve gear and were classified D7 (N-4-d and N-4-e) and D9 (N-4-c); two received Young valve gear and were classified D11 (N-4-f). All simpled locomotives had their boiler pressure reduced to 180 lbf/in2 (1.241 MPa)
Both GT and CN took some of these rebuilding efforts out of their original numerical sequence. CN numbered class N-4-a locomotives from 2525 through 2660 and numbered the alternative rebuilding classes from 2661 through 2686. The rebuilt simplified locomotives remained in freight service until the final replacement of steam with diesel locomotives.[1]
Clegg, Anthony; Corley, Ray (1969). Canadian National Steam Power. Montreal: Trains & Trolleys.
Edson, William D.; Corley, Raymond F. (Autumn 1982). "Locomotives of the Grand Truck Railway". Railroad History (147). The Railway & Locomotive Historical Society, Inc. ISSN0090-7847.