The original 1875 Union Station was built to serve 5 lines, which had 4 separate stations. (The Worcester and Shrewsbury Railroad, a narrow gauge line, used a separate terminal nearby until it was converted to a trolley line around 1900.) The 1911 Union Station, located slightly west, eliminated most grade crossings and improved freight interchange.
The east-west tracks (Boston and Albany Railroad) had a side platform plus an island platform. (The island served the 2nd and 3rd tracks south of the station; the new island in planning will be slightly north and serve the 1st and 2nd tracks).
There were 5 tracks on the northwestern side of the station, with two islands and a side. The three western stub tracks were through tracks; they connected the B&M lines to the north (the Boston, Barre, and Gardner to Gardner, and the Worcester, Nashua, and Rochester to Ayer) with the New Haven lines (the Providence & Worcester and the Norwich & Worcester), which crossed under the B&A west of the station. The two eastern tracks were stub tracks for the New Haven lines. Two of the through tracks still exist; the stub tracks and the westernmost through track are gone.
The first service to be discontinued was to New London on the N&W in 1946; it had been only used for Maine expresses for a while. Local service resumed in 1952 with a single RDC
. Local service to Gardner and Ayer ended in 1953, followed by Providence around 1957. The State of Maine ran via Providence, Worcester, and Ayer until 1960. New London service lasted until A-Day (May 1, 1971).