By 1912, the New Haven operated more than 2,000 miles (3,200 km) of track, with 120,000 employees, and practically monopolized traffic in a wide swath from Boston to New York City.

[3][4] An initial merger attempt between the two in 1870 was rejected by the Connecticut General Assembly, largely over fears that the merged railroad would form a monopoly.

[8] The company later leased more lines and systems, eventually forming a virtual monopoly in New England south of the Boston and Albany Railroad.

[12] Morgan and Mellen achieved a complete monopoly of transportation in southern New England, purchasing other railroads and steamship and trolley lines.

In 1914, 21 directors and ex-directors of the railroad were indicted for "conspiracy to monopolize interstate commerce by acquiring the control of practically all the transportation facilities of New England.

In 1925, the railroad created the New England Transportation Company as a subsidiary to operate buses and trucks on routes where rail service was no longer profitable.

"[16]: 862  The railroad emerged in September 1947 under a reorganization plan approved in federal court, without the vast majority of its previous non-railroad interests, and with a number of unprofitable passenger operations on marginal branches replaced with bus service.

The demise of the New Haven was likely hastened by the 1958 opening of the Connecticut Turnpike, largely paralleling the railroad’s mainline across the state, and the subsequent construction of other interstate highways.

With decades of inadequate investment, the New Haven could not compete against automobiles or trucks.In 1954, the brash Patrick B. McGinnis led a proxy fight against incumbent president Frederic C. "Buck" Dumaine Jr., vowing to return more of the company's profit to shareholders.

Knoll employed architect Marcel Breuer to design the interiors and exterior styling of the three experimental trainsets – the Dan'l Webster, John Quincy Adams, and Roger Williams – that were ordered in 1955.

Asked by the Connecticut Public Utilities Commission in February 1960 if the company's survival was in imminent danger, the New Haven's comptroller replied, "Yes, even with the best of management".

[21] Continuing financial problems forced the New Haven into bankruptcy on July 7, 1961, and federal court judge Robert P. Anderson assumed trusteeship.

Company president George Alpert blamed "government subsidies direct and indirect to our competitors, and inequitable taxes" for the railroad's deficits, pointing to billions of dollars in federal funding for highways and airports.

[23] A substantial portion of the former New Haven main line between New York and Boston was transferred to Amtrak in 1976 and now forms the northern leg of the electrified Northeast Corridor, hosting high-speed Acela Express and regional rail service.

The Connecticut Department of Transportation has painted its diesel commuter rail locomotives used on the non-electrified Danbury and Waterbury Metro-North branches, as well as its Shore Line East operation, in the "McGinnis Scheme," composed of white, black, and orange-red stripes with the iconic NH logo.

NH introduced ideas for passenger rail travel, including early use of restaurant and parlor cars in the steam era, and more during the transition to diesel.

An audacious experiment was the UAC TurboTrain, which with passive tilt, turbine engines and light weight attempted to revolutionize medium—distance railway travel in the U.S.

Passengers rode extra trains from Springfield, Boston, and especially New York to the New Haven Union Station, where they transferred to trolleys for the 2-mile (3.2 km) ride to the Bowl.