The roadway is seven lanes wide between the shortest (439 ft; 134 m) span and the center to accommodate the now-unused toll plaza.

[10] Electric streetcars replaced the horsecars on the bridge in the early 1890s, with all-electric service effective March 13, 1893.

It constructed the line from Milk Row station around Charlestown to a new freight terminal built on filled land in the Mystic River between the two channels.

[15] The railroad's construction of transatlantic port facilities, including a grain elevator and coal depot, along with additional crossings of the bridge road brought the issue to a head in 1892.

[16] That June, the state legislature passed an act authorizing Chelsea to pursue elimination of the grade crossings.

[19] Masonry piers spaced 70 feet (21 m) apart supported the iron viaduct, which cost $600,000 (equivalent to $14,200,000 in 2023) to construct.

[30] As part of the $425,000 project (equivalent to $9,600,000 in 2023), the wooden pile approaches were raised to eliminate a slope from the Charlestown viaduct.

[38]: 12  On June 27, 1934, traffic on the north draw was restricted to vehicles under 6 short tons (5,400 kg) except for streetcars, using only one lane in each direction.

[42] Initial plans had called for streetcar service by the Eastern Massachusetts Street Railway (successor to the Lynn and Boston) to be maintained during construction.

[43][44] The bridge reopened on December 23, 1935, without streetcar tracks; the Eastern Mass continued its bus operations.

[45][5]: 231  The Boston Elevated Railway began Chelsea Square–City Square bus service over the bridge on July 2, 1936.

The bridge would then become part of the state highway system to be maintained and operated by the department free of tolls.

[49][50] On January 28, 1951, six people in a wedding party died when they drove onto the Charlestown end of the closed bridge and fell into the river at the open north draw.

[54] A new fixed bridge was built near the alignment of the former south draw in 1982 as part of a project to remove trucks from neighborhood streets in Charlestown.

[57] In 1967, the Mystic River Bridge was renamed in honor of Maurice J. Tobin, former Boston mayor and Massachusetts governor.

[citation needed] Legislation was passed to transfer the bridge from Massport to the new Massachusetts Department of Transportation, effective January 1, 2010.

[63] On January 4, 1990, racial hoaxer and double murderer Charles Stuart committed suicide by jumping from the bridge.

[64][65] MassDOT employees are thus trained to monitor security cameras and watch for any vehicles that stop on the bridge for whatever reason.