In 1843, John A. Poor of Portland, Maine, advocated building a railway from Montreal to Portland in conjunction with a railroad through Maine east to Saint John, New Brunswick, and Halifax, Nova Scotia. This resulted in the formation of the Atlantic & St. Lawrence Railroad in Maine, which is today known as the St. Lawrence & Atlantic Railroad. The railroad was opened for traffic on July 18, 1853, and was taken over by the Grand Trunk the same year.

In a Feb. 12, 1845, memorandum, John Alfred Poor wrote about this daring trip to Montreal from Portland in a fierce blizzard to promote the building of the Atlantic and St. Lawrence Railroad. 

Construction started July 4, 1846. The first section from Portland to Yarmouth opened on July 4, 1848. Furhter extensions opened to Danville in January 1849 and to Bethel in March 1851. Sections into and within New Hampshire opened to Gorham on July 23, 1851 and Northumberland on July 12, 1852, and the full distance to Island Pond, Vermont, on January 29, 1853.

Grand Trunk enlarged their waterfront facilities at Portland by purchasing land from Henry Wadsworth Longfellow.