Series consists of records created and/or maintained by Grand Trunk properties in Ontario. Besides the four systems absorbed by the Grand Trunk, i.e. the Great Western (RG30-I-F), the Northern (RG-30-I-G), the Midland (RG30-I-H) and the Canada Atlantic (RG30- I- I), all of which were mainly in Ontario, the Grand Trunk acquired or had an interest in other Ontario railways and properties.
The earliest was The Toronto and Guelph Railway Company (RG30-I-C-1) which was one of the original components of the Grand Trunk "Main Line" being absorbed in 1852. The Buffalo, Brantford and Goderich Railway Company (RG30-I-C-2) was the predecessor of the Buffalo and Lake Huron Railway Company (RG30-I-C-23), the operation of which was taken over by the Grand Trunk in 1864.
The Grand Trunk, Georgian Bay and Lake Erie Railway Company (RG30-I-C-5) was initially created by an amalgamation of The Stratford and Huron Railway Company (RG30-I-C-3), The Port Dover and Lake Huron Railway Company (RG30-I-C-4) and The Georgian Bay and Wellington Railway Company (RG30-I-C-24). The South Norfolk Railway Company (RG30-I-C-6) was purchased in 1888, and became part of the Grand Trunk in 1889.
In addition to Companies that actually carried out all or part of the construction or operations for which they were chartered, the Grand Trunk sponsored or had some interest in others that for various reasons did not proceed. Some Companies of this type whose records passed to the Grand Trunk are Great Western and Lake Ontario Shore Junction Railway Company (RG30-I-C-7), Ontario and Pacific Junction Railway Company (RG30-I-C-8), The Yorkville Loop Line Railway Company (RG30-I-C-9), The Ontario Sault Ste. Marie Railway Company (RG30-I-C-10), The Kingston, Smiths Falls and Ottawa Railway (RG30-I-C-13), The Burk's Falls and French River Railway Company (RG30-I-C-20), The Erie, London and Tillsonburg Railway (RG30-I-C-21) and The Ottawa, Rideau Valley and Brockville Railway Company (RG30-I-C-22).
A much more successful railway despite its small size was the Thousand Island Railway Company (RG30-I-C-11) which was purchased in 1911. On the other hand, The Toronto Beltline Railway Company (RG30-I-C-15), though completed, was a failure. Small lengths of line were built and acquired by the Grand Trunk under the charters of The Oshawa Railway Company (RG30-I-C-14), Waterloo Junction Railway Company (RG30-I-C-16) and The Maganetawan River Railway Company (RG30-I-C-18).
The Grand Trunk wholly owned and controlled The St. Clair Tunnel Company (RG30-I-C-12) and associated with other railways in the ownership of The Toronto Terminals Railway Company (RG30-I-C-17) and The Ontario Car Ferry Company (Limited) (RG30-I-C-19).
Cartographic material (RG30M 1996-02259-8) consists of yard plans for: Sarnia, Wiarton, Lindsay, Hamilton, Toronto, Barrie, Guelph, Cardwell Jct., Atherly Jct., Beach Road (Burlington Bay), and Peterborough; and location plans for: the St. Clair Tunnel and the Bay of Quinte Railway. Plans are dated 1883-1902.