The railway from Portland to Montreal was standard-gauged in September, 1874.During the week preceding the change, each section foreman made sure all ties on his section were properly adzed and clear of gravel. The line was proposed as a connection between Portland and Sherbrooke, Quebec in 1844 by Portland entrepreneur John A. Portland was desperate to connect its ice-free port with Montreal, and Maine was at risk of being eclipsed by a similar proposal running from nearby Boston, Massachusetts.

The increased traffic from Portland and Point Levi to Montreal placed significant demands on the small train ferry (car float?

) service across the Saint Lawrence River at Montreal, and this was replaced by the Victoria Bridge by 1860.

The line was originally built to the Portland gauge of ). The first section, from Portland to Yarmouth, opened July 20, 1848.

Further extensions up the Royal River to Danville (now Auburn) opened in October, 1848, and to Mechanic Falls in February, 1849.

Spikes were laid out beside each tie, and some sidings were re-gauged before the main line.

Two eight-man squads were assigned to each five-mile section.

The GTR line to Portland was built during the boom period for New England textile mills, and various mill towns in northern New England soon saw an influx of French Canadian workers who quickly found work in the region.

Grain elevators were constructed at Portland to facilitate storage and loading of Canadian wheat for export.

Grand Trunk purchased 200 standard gauge locomotives (including 62 from Portland Company) and converted 135 old locomotives.