[3] The station was built with 4 tracks serving 2 island platforms; these were connected by two loops, allowing streetcars from the south and west to reverse direction and return to the portals and surface routes.

Boylston and Park Street were built with rectangular stone headhouses designed by Edmund M. Wheelwright that did not aesthetically match the Common.

[6][7]: 22  Signalmen used metal picks to complete a circuit, lighting up a numeral indicating which berth the car arriving on a given route would stop at.

The platforms at Park Street were retrofitted with raised wooden sections to allow elevated trains to run on the outer tracks to the Pleasant Street Portal, while streetcars continued to use the inner tracks and inner loop, entering the subway from the Public Garden incline.

[3] This arrangement lasted until Main Line service was moved to a separate tunnel under parallel Washington Street.

[11][12][13] Construction of the Dorchester Tunnel begun on May 3, 1912; this extension opened to Washington on April 4, 1915, and in three more segments to South Station Under in 1916, Broadway in 1917, and Andrew in 1918.

[3] Overcrowding had been a problem at Park Street since the Main Line was routed through it, and did not improve when it reverted to streetcar-only operations.

The problem worsened with the influx of passengers transferring at Park Street Under, as the only entrances to those platforms were narrow staircases connecting through the streetcar level.

[16] The Park Street Information Booth on the upper level, which soon became a Boston landmark, opened on December 15, 1923, and replaced an earlier kiosk in the same location.

[7][17][18] On January 3, 1925, an explosion of celluloid film carried by a passenger injured dozens on a streetcar arriving at the station.

[19] Although the 1914–15 renovation increased the platform area, the narrow stairways to the lower level still impeded passenger flow.

[27] These improvements reduced the amount of room available on the north ends of the streetcar platforms, and boarding areas were moved south.

In July 1922, the Boston Transit Commission proposed the addition of a side platform adjacent to the outer northbound streetcar track, with a passageway under Tremont Street leading to two new entrances at Temple Place.

[28] Opening the Winter Street Concourse to passengers was also proposed, but was soon rejected by the Department of Public Utilities in favor of continued use as storage space for adjacent businesses.

In 1966, the south headhouse on the southbound platform was converted to an information booth for the new Freedom Trail, with only an exit provided from the station.

[38][39] In 1967, as part of a general system rebranding by the newly formed MBTA, the subway lines were assigned colors.

[41] As part of that project, the MBTA investigated the feasibility of connecting Essex, Park Street, Washington, and State with pedestrian tunnels.

The modernization included new wall tiles, new lighting, rebuilt staircases, and upgrades to the faregate area on the southbound platform.

[47] The agency obtained a $22 million federal grant in 1984 to partially fund the Central, Park Street, and Washington station work.

One work by Christopher Janney featured a synthesizer controlled by photoelectric sensors, placed so that passing passengers would interrupt the beams and change the sounds.

[50] The MBTA opened bidding for an additional round of construction in March 1991; a $6 million contract was issued on July 10.

[54] Around that time, a new south headhouse was built on the southbound side as the Freedom Trail information center was moved to a new structure nearby; it was exit-only until automated fare collection equipment was installed around 2007.

[55][56] Around 2000, the MBTA outfitted the Green Line platforms with portable lifts as a temporary accessibility measure to serve the new low-floor Type 8 streetcars.

[65][66] The elevator between the Red Line lobby and platform will be replaced beginning in 2025 as part of accessibility improvements at Downtown Crossing.

[73] Benedictions by Ralph Helmick consists of two giant bronze hands each mounted separately over the inbound and outbound Red Line tracks on the lower level of the station.