Major communities served include Portland and Lewiston in Maine; Berlin, New Hampshire; Island Pond, Vermont; and Sherbrooke and Montreal in Quebec.

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.

Writer, critic, and Atlantic and St. Lawrence Railroad investor, John Neal wrote of the necessity "to drive Boston out of the business and secure [a] monopoly.

Four months later, on August 5, 1853, the Grand Trunk Railway leased the two companies, giving the Toronto-Montreal line an extension east to Portland.

Grand Trunk equipped approximately 1,000 freight cars with experimental "sliding-wheels" in 1863 at company shops in Sarnia, Ontario, and Pointe-Saint-Charles in Montreal.

Gauge could be adjusted by removing and inserting axle pins on special tapered-gauge track segments at interchange points.

Fourteen steamship lines were serving the Grand Trunk wharves at Portland by 1896 with connections to Bristol, London, Liverpool, Glasgow, and Antwerp.

Annual car loadings in 1973 were 12,758 for Berlin, 5,794 for Groveton, and 1,161 for Mechanic Falls; but the Boston and Maine Railroad carried some of the traffic for the New Hampshire mills.

[4] Dressed meat from Chicago to Maine continued to use the shorter Canadian routing as long as railway reefers remained competitive with highway trucking.

The CNR class U-1 4-8-2 locomotives pulling as many as 17 car trains around Dominion Day would be serviced at Rigby Yard in South Portland before making the return trip.

[2] Despite the decline in traffic being handled over the line, its strategic connection to the Atlantic Ocean for Montreal saw other use arise during World War II.

[6] The Portland–Montreal Pipe Line was built to carry oil from terminals in South Portland to refineries in Montreal; the pipeline followed the GTR route along certain parts and is still in use today.

Wharves at Portland were used by the United States Navy as Casco Bay became destroyer base Sail during the Battle of the Atlantic.

[7] Grand Trunk Piers housed a Navy supply pier and training schools for combat information center (CIC), night visual lookouts, surface and aircraft recognition, search and fire control radar operators, gunnery spotting, anti-aircraft machine guns, and anti-submarine warfare (ASW) attack.

In 1989, the St. Lawrence and Atlantic Railroad was formed to take over operation of the Island Pond-Portland section, and several years later this was extended to the border at Norton.

In April 2012, the Maine Department of Transportation put a project out to bid which would "purchase, design, and construct a portion of rail line for future passenger service to Lewiston and Auburn.

As of 2013[update] prospective operators of a night train from Montréal to Boston are attempting to get access to the St. Lawrence and Atlantic right-of-way for a 2014-passenger launch.

The preliminary opinion is that the collapse was due to an ice jam and rapid water level rise following an unseasonal day of rain.