The St. Lawrence & Atlantic (reporting mark SLR) currently operates the oldest line going from Canada to the U.S.  Built in 1853 in 5' 6" broad gauge, it was a joint effort between the Atlantic and the St. Lawrence (actually two "sections", like the current SLQ and SLR) leading from Montreal, Quebec, to Portland, Maine.  Soon after the line was built, the Atlantic & St. Lawrence was leased to the Grand Trunk Railway, with the line retaining broad gauge format until 1874.  In 1880, the Grand Trunk Railway extended their line to Chicago, which provided a route from the American Midwest to Montreal (the "St. Lawrence") and Portland (the "Atlantic").  When Grand Trunk became part of Canadian National in 1923, the line was then owned by CN.  The "Atlantic & St. Lawrence" corporate name was dropped in 1959.

The St. Lawrence & Atlantic Railroad Company has existed since 1989, when it acquired the American half of the line (from Norton, Vermont, to Portland).  In 1998, CN announced that its Sherbrooke subdivision, from Richmond, Quebec to Norton, was up for sale.  Initial fears of abandonment subsided when the SLR took over that section of the line in November 1998.  To differentiate between the American and Canadian sides of the line, the section from Norton to Richmond was dubbed the "St. Lawrence & Altantic Quebec" (SLQ).  CN's Zebra-striped paint scheme was replaced by SLR's bright yellow and black colours (with bilingual text on opposite sides of the locomotives) and a variety of leasing colour schemes.

Much is made of the fact that this is the only line capable of hauling doublestacks from Montreal to Portland, making it a very important link for container transportation.  CN still owns the line from Richmond to Montreal, and the SLR interchanges with CN in Richmond.

Early 2002 "big news" was the purchase of Emons Transportation, the parent company of St. Lawrence & Atlantic, by Genesee & Wyoming.  The future looked bright, and things seem now to be going well.  Four pairs of road slugs in the 800- and 3800-series and a fleet of GP40's in the 3000- and 3200-series provide motive power for over-the-road freights.  Average train lengths on the main 393 and 394 daily freights are between 70 and 80 cars.