Boston and Lowell Railroad station 1852 (BPL).
First Boston and Lowell Railroad station, 1852, BPL (click for larger image).
(Edited to add information, 1/10/13)
The first Boston and Lowell depot sat alone on the waterfront, well back from Causeway street, seen here at the lower right corner of the map. The depot is at the top center of the map above. The footprint of the depot in the map doesn't jibe with the print of the building above, which appears to be L-shaped, but there you go.
The company was chartered in 1830, but the track was not finished until 1835. It followed the general path of the Middlesex canal that had preceded it, and was used to haul the same freight between Boston and the Lowell mills. The first runs stopped only at Lowell and Boston; it took petitions from people living along the route to get the company to build stations along the way.
New Boston and Lowell station, Causeway street (BPL).
New Boston and Lowell station, opposite view (BPL).
The two photographs above show the second station, which sat on Causeway street with the Eastern and Fitchburg line depots. The old depot was kept and used for freight. All on the lines had both passenger and freight depots, though I've been focusing on the passenger facilities in this series. I don't have a date for these photographs, but the Second Empire style of the roofing suggests the early 1870s. The map below shows the new depot in place in 1883.
Boston and Lowell station, Causeway street, 1883. Note in the upper left, the old terminal is now a freight depot. During the 1880s, the Boston and Lowell would be leased by the Boston and Maine, and cease to be an independent company.
Resource: Boston's Depots and Terminals, Richard C. Barrett.