The International Railway of Maine was a historic railroad constructed by the Canadian Pacific Railway (CPR) between Lac-Mégantic, Quebec, and Mattawamkeag, Maine, closing a key gap in the railway's transcontinental main line to the port of Saint John, New Brunswick.

The CPR completed its route from Montreal, Quebec, to Vancouver, British Columbia, in 1885. In the decades prior to the use of ice-breaking ships in the Gulf of St. Lawrence and St. Lawrence River, the port of Montreal was closed from December to May, limiting any advantage that the railway might have over its competitors.

CPR's primary Canadian competitor, the Grand Trunk Railway (GTR), managed to avoid the winter ice problems in Montreal by using the ice-free port of Portland, Maine, accessed by a route constructed by the St. Lawrence and Atlantic Railroad which the GTR had purchased in the mid-1850s.

The Delaware and Hudson Railway ran a feeder route down the valleys of Lake Champlain and the Hudson River to New York City. The Maine Central Railroad operated an arduous route over the White Mountains from St. Johnsbury, Vermont, to Portland.

Looking 350 miles directly east from Montreal however, CPR surveyors saw the Canadian port of Saint John, New Brunswick, was underutilized and Saint John was accessible by a route across northern Maine which was less mountainous than other options for reaching the Atlantic coast.

Some sections of a direct railway route between Montreal and Saint John already existed in the 1880s:

A roughly 100 mile / 160 km gap between Mattawamkeag and Megantic required new construction to complete the Montreal-Saint John direct route.

The CPR acquired the International Railway in the mid-1880s and surveyed a line running directly from Megantic to a point on the E&NA (then leased by the Maine Central) at Mattawamkeag. This portion of new railway would cross the International Boundary between Megantic, Quebec and Jackman, Maine, thus the CPR organized two separate companies: