Streetcar congestion in downtown Boston led to the subways in 1897 and elevated rail in 1901.
The Tremont Street subway was the first rapid transit tunnel in the United States.
Grade-separation added capacity and avoided delays caused by cross streets.
The first elevated railway and the first rapid transit line in Boston were built three years before the first underground line of the New York City Subway, but 34 years after the first London Underground lines, and long after the first elevated railway in New York City; its Ninth Avenue El started operations on July 1, 1868 in Manhattan as an elevated cable car line.

Various extensions and branches were added at both ends, bypassing more surface tracks.
As grade-separated lines were extended, street-running lines were cut back for faster downtown service.
The last elevated heavy rail or "El" segments in Boston were at the extremities of the Orange Line: its northern end was relocated in 1975 from Everett to Malden, MA, and its southern end was relocated into the Southwest Corridor in 1987.
However, the Green Line's Causeway Street Elevated remained in service until 2004, when it was relocated into a tunnel with an incline to reconnect to the Lechmere Viaduct.
The Lechmere Viaduct and a short section of steel-framed elevated at its northern end remain in service, though the elevated section will be cut back slightly and connected to a northwards viaduct extension in 2017 as part of the Green Line Extension.
Streetcar congestion in downtown Boston led to the subways in 1897 and elevated rail in 1901. The Tremont Street subway was the first rapid transit tunnel in the United States. Grade-separation added capacity and avoided delays caused by cross streets. The first elevated railway and the first rapid transit line in Boston were built three years before the first underground line of the New York City Subway, but 34 years after the first London Underground lines, and long after the first elevated railway in New York City; its Ninth Avenue El started operations on July 1, 1868 in Manhattan as an elevated cable car line.
Various extensions and branches were added at both ends, bypassing more surface tracks. As grade-separated lines were extended, street-running lines were cut back for faster downtown service. The last elevated heavy rail or "El" segments in Boston were at the extremities of the Orange Line: its northern end was relocated in 1975 from Everett to Malden, MA, and its southern end was relocated into the Southwest Corridor in 1987. However, the Green Line's Causeway Street Elevated remained in service until 2004, when it was relocated into a tunnel with an incline to reconnect to the Lechmere Viaduct. The Lechmere Viaduct and a short section of steel-framed elevated at its northern end remain in service, though the elevated section will be cut back slightly and connected to a northwards viaduct extension in 2017 as part of the Green Line Extension.
Massachusetts Bay Transportation Authority