When the first subway car pulled out of Park Street Station, it may have seemed to Bostonians as though underground transit was the obvious solution to the city's growing traffic congestion. In fact, there were several other proposals, including streetcars using open trenches to access the Common.

When the Boston Transit Commission determined that a subway system would be best, the legislature appropriated $5,000,000; construction began on March 28, 1895.

To complete the daunting task, the engineers combined the deep underground "tube" style of construction pioneered in London with the open excavation method used in Paris. In this new "cut-and-cover" technique, workers supported the walls of a freshly dug trench with temporary wooden braces. Once the trench was deep enough, the walls and floor — made of concrete, steel beams, and waterproof grout — were added. Finally, the roof of the tunnel was made by building brick arches between steel support beams. The upper surface was then finished with a layer of concrete and soil. This system would soon become standard practice in nearly all American subway construction.

The process did not go entirely as planned. The first section was laid out along the edge of the Old Common Burial Ground. Workers saw only a few gravestones as they began digging, but they would eventually unearth the remains of over 900 unmarked graves, an unsettling discovery. The Boston Post jumped on the story, running it under the headline "Hideous Germs Lurk in Underground Air" with an illustration of a large, scary-looking "subway microbe." The pastor of the Park Street Church,called the subway "an infernal hole" and "an un-Christian outrage" when a water main ruptured, coating his office with mud. "Who?" he asked from the pulpit, "is the Boss in charge of the work? It is the Devil!"

Despite this and other setbacks, including an explosion at Boylston and Tremont Streets that killed nine men, construction continued, and the project was finished early and under-budget. The Boston Globe reported "nearly everything went as smooth as the proverbial clockwork, and the opinion heard on all sides was that, as far as it goes, the subway is an unqualified success." The best part was the lack of congestion on Tremont Street due to "the ease and speed with which the hitherto snaillike electrics on Tremont Street now glide through the underground passage."

The Boston Elevated Railway Company, which built and operated the system, quickly integrated the new underground trains with the elevated lines it was building at the same time. When the Company's Main Line Elevated opened in 1901, it connected to the Tremont Street subway, creating an efficient system of public transportation in downtown Boston.

It was not long before other American cities followed Boston's example. New York opened nearly nine miles of subway in October 1904. Philadelphia's system was built between 1905 and 1908; it combined underground, surface, and elevated lines just as Boston's did.

The Tremont Street Subway was only the beginning. Although the growing popularity of the automobile would eventually cut into ridership on the El and the subway, the T would remain one of Boston's greatest assets. By the end of the 1970s, the T was transporting 300,000 passengers daily. Today the T serves over 1.3 million passengers daily across the system.