The majority of streetcar lines in Southern New England ran in mixed traffic on city streets in downtown areas and alongside local highways between towns. Only a smaller number of lines had significant interurban characteristics, including long stretches of private rights of way and roadside reservations that supported operations at speeds far higher than street traffic; most were located in relatively uncrowded Eastern Connecticut with longer distances between population centers.
Currently, the only services that come close to the definition of an interurban are two sections of the MBTA rapid transit system in Boston, both converted commuter rail routes rather than original interurbans. The Green Line D branch (formerly the Boston & Albany Railroad's Highland branch) was converted in 1959 and runs modern light rail equipment; the Ashmont–Mattapan High Speed Line was converted in 1929 and runs historic PCC streetcars.