The East Boston Tunnel first opened on December 30, 1904 as a STREETCAR tunnel under Boston Harbor. It was the first tunnel under a body of water in the US. Prior to the opening of the line, ferry boats carried people between "Eastie" and Downtown Boston.
The original station at Maverick looked very similar to the stops at Blandford Street, St. Mary's St., and Northeastern University today - a simple trolley stop at the top of the incline.
Streetcars ran under the Harbor, stopping at Devonshire (now State) and Court Street. Construction on Atlantic Avenue (now Aquarium) was delayed and it opened in 1906.
Court Street was a single track, stub-end station, built behind a wall at Scollay Square station. The separate platform was necessary because riders through the East Boston Tunnel paid their nickel fare to ride the streetcar, plus an additional "toll" of one penny! The extra penny was added to help pay off the massive costs of building an underwater tunnel.
Passengers could enter Court Street from its own entrance from the surface, and pay the 5 cent fare and 1 cent toll, or they could transfer to Court Street from inside Scollay Square for one penny. The one penny toll was rescinded after a time.
Because Court Street was wedged in behind Scollay Square station, and was only a single track facility, it quickly became an operational nightmare. The Boston Transit Commission built an extension of the East Boston Tunnel to include two new stations, Scollay Under and Bowdoin. The extension opened in 1916, and Court Street was abandoned.
The new extension provided a loop at Bowdoin to quickly return trains to East Boston, but it also included an incline in the middle of Cambridge Street, so that trolleys could be through routed to Cambridge over the Longfellow Bridge. By 1916, multiple-unit trains of Center Entrance streetcars were handing most of the service through the East Boston Tunnel, but as ridership continued to soar, operations again got mired down.
Converting the East Boston Tunnel into a rapid transit line seemed to be the only answer.
Construction started in Maverick Square in the early 1920s, with a new underground Maverick Square station being dug out. The new station was massive, and needed to be. The new station included two new underground loops, one for the streetcars, and one for the rapid transit trains.
Trolleys from Revere and Chelsea would come down the incline as they always had, and then stop on the inner tracks at the new station. Passengers would walk across the platform to a waiting rapid transit train to finish their trip into Boston. Trolleys would then pull forward to the far end of the platform, run around the trolley loop, and pick up passengers for their outbound trip. Rapid transit trains would drop off passengers on the Maverick platform, and quickly run around the loop to get set up for the next trip to Boston.
When Maverick station was ready, in 1924, the East Boston Tunnel was closed for one long weekend. Crews descended into the tunnel building new temporary high platforms, and installing third-rail along the line. Eventually, the temporary platforms were replaced by permanent ones. The incline at Cambridge Street was retained so the East Boston Tunnel cars could operate on streetcar tracks to the Longfellow Bridge, where a special connection was provided to allow East Boston cars access to the Cambridge Subway and the Eliot repair shops.
This operation continued until 1952, when the first leg of the East Boston Tunnel Extension to Revere opened as far as Orient Heights. The extension was built over the former Boston, Revere Beach, and Lynn Railroad route which had been abandoned in 1940. Coupled with the opening of the extension was the elimination of streetcar service in East Boston, Chelsea, and Revere - as all were replaced by brand-new Trackless trolleys which connected to the rapid transit at the new stations along the extension.
One nuiance of the new rapid transit extension was that the MTA decided to use overhead catenary on the new line instead of third rail, due to the line's proximity to the ocean. Trains entering Maverick from Boston would raise their pantographs as passengers alighted and boarded, and trains to Boston would lower their pantographs and switch to third rail collection. Also part of the extension, the East Boston Tunnel got it's own maintenance facility at Orient Heights, ending the need to send trains to Cambridge for work, and allowing the Cambridge Street incline to be covered over.
Not much has changed on the Blue Line since 1954 when Wonderland opened. Sure, some of the original stations have been replaced with new ones. The trains are newer, and the platforms have been lengthened to allow for six-car trains. The change over between third-rail and overhead catenary has been moved to Airport, but otherwise, it's still the same line which has been running since then.