It wasn't just the freeways that were put underground. Boston had an extensive network of "elevated" rapid transit lines, which had been built (100 years ago) on steel structures over main radial streets (and which completely blight such streets). Several US cities went for this at the time. Manhattan cleared them out in the 1950s (they remain in other New York boroughs, along with several other US cities), but Boston has got rid of most of them underground as part of this project.
Hmmm, check your timeline. The last of the old elevated sections of line, the Orange Line's Washington Street El, was torn down in 1987. It wasn't part of the Big Dig, though some stations on the Orange Line were renovated as part of the project.
The MBTA green line used to be elevated between Haymarket and Science Park (where it crosses the Charles). As part of the Big Dig, it was put underground.
Crossing the Lechmere Viaduct, a long archeed structure, takes the line southward, across the Charles River Dam, and past the Museum of Science. Science Park is at the end of the viaduct as the Green Line enters elevated trackage. The Science Park Station is two platforms on either side of the two tracks.1972 photo of the Viaduct, looking toward the famous Boston Garden (home of the Boston Celtics (NBA) and Boston Bruins (NHL))2000 photo of North Stationdemolition of the viaduct to Science Park... the TD Garden, the replacement arena for the old Garden, is visible in the background
The elevated section takes the line over the city streets in the area of North Station, and is the last "EL" operated by the MBTA. The line takes a sharp turn east, to the North Station stop. Passengers can transfer to the Orange Line or commuter rail from this station. It is also the site of the old Boston Garden, now demolished. This entire area has seen some major changes recently as the Green Line is being moved to an underground tunnel from Haymarket to Science Park. This station was one of the more unique on the Green Line as it featured both an elevated station, and also a surface level platform on the street. Trains that would terminate at North Station would use the surface station, trains continuing on toward Lechmere use the upper level. Continuing, the Green Line turns sharply back to the south and descends an incline into the subway at Haymarket Portal. Now the track alignment got a little complicated. At the mouth of the portal is where the North Station surface tracks diverged from the elevated. The surface line heading north, stays at ground level, surrounded by the two inclines to the elevated portion. The surface line tucked underneath the elevated structure, and traveled a very short distance to the surface level North Station stop. This was once a turnback loop, but was trimmed back to a stub end track. This surface stop was closed in 1997 as part of the Green Line relocation project. The old inclines and Haymarket portals have been removed. The Green Line now uses a temporary incline to reach the elevated trackage, using a new portal constructed in the area once used by the old Orange Line El. This incline is on a lower level, of a double deck structure. The upper level is a temporary highway off ramp. The tracks swing west and connect to the old elevated structure.
More changes are in store for this area as the Green Line relocation project and Central Artery "Big Dig" highway project move ahead. The relocation project will move the Green Line into a subway as well as the highway. It will spell the end of service on the elevated structure around North Station. It is planned that the new Green Line subway will run beneath North Station, exiting out of a new portal to take the line to the Lechmere Viaduct. The new North Station stop will be a "superstation", featuring shared inbound platforms for Green Line and Orange Line trains.
The current subway portals were once shared with the Orange Line elevated to Charlestown. When the Green Line relocation is completed, the Haymarket Portals will be closed permanently.
- also being demolished is the old ramp to I-93's Charles River crossing, one of the major bottlenecks in Boston traffic.
Some pre-Big Dig videos:1996: northern entrance to the Dewey Square Tunnel1988
(video is from a Saturday or Sunday afternoon)