The first underground rapid transit tunnel in North America opened in Boston on September 1, 1897. Streetcars ran into the Tremont Street Subway that morning, and crowds gathered at the Public Garden end to watch them come back out of the portal.
The tunnel was built to solve a traffic problem, not to make a statement. Boston's streetcar network had electrified in the late 1880s, which made the cars faster and more numerous, and funneled them all through a downtown laid out in the colonial period. Tremont Street was the worst of it, carrying on the order of 10,000 streetcars a day across tracks so crowded that the whole street jammed from morning to night. Photographs taken for the Boston Transit Commission in 1895 show a solid block of cars outside the Park Street Church. Horse-drawn fire engines and ambulances could not get through.
A commission report in 1892 laid out the options, and the legislature chartered the Boston Transit Commission in 1894 to build the subway. Construction started in March 1895. The method was cut-and-cover: dig a trench, build the tunnel in it, put the street back on top. Workers had to keep traffic moving above them the whole time and reroute sewers, gas mains and telephone lines underneath.
The line was short. The first two stations to open were Boylston and Park Street, with service continuing through the downtown section under Tremont Street. It carried streetcars rather than the heavy trains that later subways used, which is why the line later became light rail.
What the opening proved was that a dense American city could dig under itself and keep running while it did so. New York broke ground on its own subway in 1900 and borrowed heavily from the Boston experience.
The original tunnel between Park Street and Boylston is still in daily use as part of the MBTA Green Line. It has been designated a National Historic Landmark, and it is the oldest rapid transit tunnel in North America still carrying passengers.