For a short time at the end of the 19th century the CAR handled up to 40% of the grain traffic from Lake Huron; this was due to a combination of factors including the advent of the grain boom on the Canadian Prairies and prior to the construction of the Canadian Northern Railway's transcontinental line across Ontario, as well as prior to the opening of the Fourth Welland Canal.

The GTR encountered financial difficulty during and after the First World War and was dissolved and its assets merged into the Canadian National Railways (CN) in 1923.

J.R. Booth was an Ottawa-based lumber baron who in the latter half of the 19th century amassed timber rights approaching 7,000 sq mi (18,000 km2) in central and northern Ontario.

The M&OJ had received a charter to build southeast from Ottawa to Coteau, QC on the north bank of the St Lawrence River.

The C&PL had received a charter to build a bridge across the St Lawrence River to Valleyfield, QC and then across southwestern Quebec to Swanton, VT on the eastern shore of Lake Champlain.

In 1879 Booth, with William Perley of Ottawa and J. Gregory Smith of St. Albans, Vermont, purchased the lines from the Macdonald group as part of an aggressive railway expansion plan.

Despite opposition from the Canadian Pacific and from Toronto business interests, despite fires at his large Ottawa saw mill and the deaths of his partners, Booth pushed forward with his rail line.

Under pressure from the federal Department of Indian Affairs, the local band that autumn surrendered the necessary 315.5 acres (127.7 ha) for the rail access, terminus, port facilities and residences.

[17] The railway accessed Depot Harbour by crossing South Channel between Parry Island and Rose Point on the Wasauksing Swing Bridge.

In 1898 Booth created the Canada Atlantic Transit Company to operate steamships on the Great Lakes from Depot Harbour to Fort William and in 1899 the Canada Atlantic Transit Company of the United States to operate between Depot Harbour and American ports such as Chicago and Duluth.

[18] During their early years, the ships of both companies, as well as other vessels operating under charter, regularly carried goods between the upper Great Lakes and Depot Harbour.

[19] Throughout the 1890s, the Government of Canada promoted the creation of a second transcontinental railway line that could compete with the Canadian Pacific as well as American railroads.

[23] Whether it was because Booth at age 74 was tired, or because he realized that competition from other transcontinental lines would soon cause serious problems for the CAR, he did everything possible in the early years of the 20th century to make every aspect of the railway profitable, and therefore attractive to potential buyers.

[26] After the purchase closed, the GTR appeared to be in no hurry to consolidate the CAR into the larger system, operating it instead as a stand-alone company for another nine years.

While a Grand Trunk subsidiary, the Canada Atlantic purchased the Pembroke Southern Railway in 1906[27] and in 1907 the CAR created the Ottawa Terminals Railway Company; in 1912 the OTR built a large train station in downtown Ottawa near Parliament Hill and directly across Wellington Street from the GTR-owned Château Laurier hotel.

[29] After the First World War, the bankrupt (or near-bankrupt) Grand Trunk Railway was finally taken over in 1923 and absorbed into the new government-owned Canadian National, which also took over ownership of the former Canada Atlantic properties.

Despite efforts by the Grand Trunk Railway to turn a profit, including the former CAR lines, it was fairly obvious not long after the 1905 purchase that this was not going to happen.

During the run-up period to 1923, while it was well understood that the federal government would force the amalgamation of a number of money-losing railways, including the Canadian Northern and eventually the Grand Trunk, which would require extensive rationalization of trackage.

[31] The GTR found that heavily loaded eastbound trains from Depot Harbour could be routed south around the Haliburton Highlands to eastern Ontario at less expense.

[35] The CAR steamship companies that had been sold along with the railway in 1905 to the Grand Trunk continued to operate under CN ownership until after World War II.