Sub-series consists of records created and/or maintained by Canadian Northern railway companies in Ontario. These companies can be grouped into three categories.
The first is comprised of railways that formed part of the Canadian Northern's main transcontinental line. Of these the earliest was The Ontario and Rainy River Railway Company (RG30-III-E-1) which formed the Ontario section of the early line from Winnipeg to the lakehead. The James Bay Railway Company (RG30-III-E-2) was originally intended to drive north from Parry Sound to James Bay but, in the event, only a line from Sudbury to Toronto was constructed before it was reincorporated as The Canadian Northern Ontario Railway Company (RG30-III-E-3). In addition to the earlier trackage, railway lines were built from Port Arthur through Ottawa to Hawkesbury, Ontario, where it connected with the Canadian Northern Quebec Railway (RG30-III-F-8) and from Toronto to Ottawa. The Ontario And Ottawa Railway Company (RG30-III-E-4) was intended to connect the lines within Ontario but never implemented its charter.
A second group consists of formerly independent railways in Eastern Ontario acquired by Mackenzie and Mann either for their local traffic potential of because parts of their lines could be incorporated in the Canadian Northern's main line. One such was The Bay of Quinte Railway Company (RG30-III-E-7) which operated a line, constructed by its predecessors The Kingston, Napanee and Western Railway Company (RG30-III-E-5) and The Bay of Quinte Railway and Navigation Company (RG30-III-E-6) north from Lake Ontario. Another, the Central Ontario Railway (RG30-III-E-9) with its predecessor The Prince Edward County Railway Company (RG30-III-E-8) and its subsidiary, The Ontario, Belmont and Northern Railway Company, later the Marmora Railway and Mining Company (RG30-III-E-10) ran from Prince Edward County to Trenton and then drove north as a mineral and exploration line. The Irondale, Bancroft and Ottawa Railway Company (RG30-III-E-11) operated 51 miles of line in the heart of Eastern Ontario while The Bessemer and Barry's Bay Railway Company (RG30-III-E-12) had 7 miles of track in the same region. The Brockville, Westport And North-western Railway Company (RG30-III-E-13) had a line running from Brockville on Lake Ontario to Westport in the Rideau lakes.
The last group consists of street, suburban and interurban electric railways in the Toronto, Hamilton and Niagara regions. Sir William Mackenzie was a prominent entrepreneur in the field of hydroelectric power companies and of urban transport, both in Canada and abroad. Some of the companies became part of the Canadian Northern system. The Niagara, St. Catharines and Toronto Railway Company (RG30-III-E-16) which had acquired The Port Dalhousie, St. Catharines and Thorold Electric Street Railway Company (RG30-III-E-14) and The Niagara Falls, Wesley Park and Clifton Tramway Company, Limited (RG30-III-E-15) operated services in the Niagara peninsula. The Toronto Suburban Railway Company (RG30-III-E-17) and The Toronto Eastern Railway Company (RG30-III-E-18) operated lines in Toronto and to the Northwest and east of Toronto. The Toronto, Niagara and Western Railway (RG30-III-E-19) designed to link Toronto and Hamilton, was never constructed.
