The last element to be completed in the main line of the Boston & Providence Railroad, the Canton Viaduct rises 70 feet and extends for 615 feet as one of the most impressive masonry structures of its time in the United States. Thick buttresses that culminate in segmental arches supporting the cantilevered track bed connect two parallel walls five feet thick, separated by a four-foot air space. At the base, six arches perforate the wall for the east branch of the Neponset River and a larger arch for Neponset Street (in 1952, an additional arch, lined with concrete, allowed further street traffic). One of the three oldest multiple arch masonry railroad bridges in the country, the Canton Viaduct was the first in New England.