Fitchburg Depot, Causeway street (photo from Wikipedia).
Fitchburg Depot, Causeway street, on the right, 1883.
(Edited to add information 1/9/13)
The Fitchburg Railroad was founded in 1842. The line originally terminated in Charlestown, but moved across to Boston in 1848. The line crossed the mouth of the Charles river to Charlestown and northwest to Fitchburg, and was later extended west through northern Massachusetts, with branches running to Vermont and New York. Part of the line ran through the Hoosac Tunnel. The Hoosac went 4.75 miles through the Hoosac range. The work took 20 years at a cost of $21,000,000, at a time when a dollar was a dollar. At the time it was the second longest tunnel in the world, and remains the longest in the United States east of the Rockies.
The Boston depot shown above was built, quite appropriately, from Fitchburg granite, and was known a Crocker's Folly (Alvah Crocker being the president of the company) and later the Great Stone Castle. When the line moved to the north union station, the space was used by the company for offices, There was a fire in 1925, and by 1928 the structure had been demolished.
The Boston and Maine line bought the Fitchburg Railroad in 1900.
Jenny Lind Tower (Truro Historical Society).
Here's a fun story/fact. Famous Swedish singer Jenny Lind performed in the Fitchburg depot when she visited Boston. The show was oversold, and in order to satisfy unhappy ticket holders left outside, Jenny sang from one of the turrets. When the depot was torn down, a wealthy fan had the turret removed block by block and rebuilt near Highland Light in Truro MA.
Great story, but apparently not quite accurate. Lind did perform in the second floor auditorium (then the largest in New England) in 1850, and contemporary newspaper reports describe fans rushing the stage (does that sound familiar?). However, there was no mention of Lind singing from the turret. the depot was not torn down until 1927, and the lawyer who bought and moved the turret was not born until seventeen years after the concert.
Resource: Boston's Depots and Terminals, Richard C. Barrett.