That company was merged into the main B&M on March 19, 1845, and opened on July 1, leading to the abandonment of the old connection to the B&L (later reused by the B&L for its Wildcat Branch).

This was primarily the main line from Boston west via the Hoosac Tunnel to the Albany, New York, area, with various branches.

The B&M flourished with the growth of New England's mill towns in the late 19th and early 20th centuries, but still faced financial struggles.

Later, it faced heavy debt problems from track construction and from the cost of acquiring the Fitchburg Railroad, causing a corporate reorganization in 1919.

After steady growth from 1901 to 1913, passenger rail ridership around Boston peaked in 1920 and began to decline due to competition from private automobiles and service cuts during World War I.

[7]: 11  In the mid-1920s, after several difficult years, the B&M discontinued service on some marginal lines and began using small self-propelled railcars on others.

Ridership sharply increased during World War II; the B&M had a slower postwar decline than its contemporaries, though major frequency reductions occurred in 1949–1950.

The New York–Montreal Green Mountain Flyer/Mount Royal, which had Boston sections running on the B&M via Bellows Falls, ended when the Rutland Railroad discontinued all passenger service, in 1953.

[9][8]: 122  Fitchburg mainline service was trimmed from Troy, New York, to Williamstown, Massachusetts, in January 1958, and discontinued soon afterward.

From January 1963 to March 1964, the state Mass Transportation Commission funded an experiment testing various fares and service levels on the two railroads.

On August 3, 1964, the Massachusetts Bay Transportation Authority (MBTA) was formed (as an expansion of the MTA funding district) to subsidize suburban commuter rail operations.

Service via Concord to Laconia, New Hampshire and to Montreal via White River Junction ended, though a single Boston–Concord round trip remained.

[12] After out-of-district communities agreed to subsidies, service was re-extended to Ayer, Lowell, Ipswich, and Rockport on June 28.

[12] The Montrealer was discontinued in September, 1966; local service on the Connecticut River Line lasted until the end of that year.

The four routes with single daily round-trips slowly ended: South Sudbury on November 26, 1971; Newburyport in April 1976; Haverhill in June 1976; and Bedford on January 10, 1977.

)[12] On December 27, 1976, the MBTA bought all B&M commuter equipment, as well as most of the B&M's trackage on Boston's northside (including several abandoned lines).

By 1980, though still a sick company, the B&M started turning around thanks to aggressive marketing and its purchase of a cluster of branch lines in Connecticut.

This was the beginning of the end of the Boston & Maine corporate image, and the start of major changes, such as new labor issues which caused the strikes of 1986 and 1987, and drastic cost-cutting such as the 1990 closure of B&M's Mechanicville, New York, site: the largest rail yard and shop facilities on the B&M system.

Up until CSX Transportation acquired Pan Am Railways on June 1, 2022, Boston & Maine Corporation continued to exist, but only as a non-operating ward of PAR.

Boston & Maine owned the property (and also employed its own railroad police), while Springfield Terminal Railway, a B&M subsidiary created by owner Timothy Mellon to break the unions' higher wage scales,[citation needed] operated the trains and performed maintenance.

The 1935 three-car trainset known as the Flying Yankee, virtually identical to the streamlined equipment the Budd Company built for the Chicago Burlington & Quincy Pioneer Zephyr, was retired in 1957 and was then displayed at the Railroad for another 36 years.

[18] The equipment was relocated and eventually purchased by the State of Maine, but both public and private restoration efforts were unsuccessful.

[18] In November 2023, the state of New Hampshire put the equipment up for sale, with a focus on "the relocation and encouraged restoration" of the trainset.

[18]In April 2024, the trainset was sold to the Flying Yankee Association, who hopes to restore and operate the set in the Mt.