The B&M leased the Eastern in 1884, though it continued to use its own terminal.
As a condition of the B&M's 1887 lease of the B&L, the state required the B&M to construct a union station for use by the combined B&M system plus the Fitchburg.
After years of resistance by the B&M, construction on North Union Station began in 1893.
The station was built as an eastward expansion of the B&L station, with a total frontage of 568 feet (173 m) on Causeway Street.
The center of the new facade was an 80-foot (24 m)-high granite triumphal arch flanked by four massive columns.
The east side was formed by a five-story baggage and express building.
The station was designed by the firm of Shepley, Rutan and Coolidge, which designed South Station several years later.

The new station was opened in stages from August 1893 to June 1894.
The Eastern depot had been demolished in 1893 to allow construction to proceed.
The B&M depot was demolished in 1897, with the site used for the Canal Street incline of the Tremont Street subway.
The Fitchburg was leased by the B&M in 1900, after which the former Fitchburg depot was used as the B&M offices.
By that time, the station was popularly known as "North Station".
The former Fitchburg depot burned on January 17, 1925; it was demolished in 1926-28.
The B&M leased the Eastern in 1884, though it continued to use its own terminal. As a condition of the B&M's 1887 lease of the B&L, the state required the B&M to construct a union station for use by the combined B&M system plus the Fitchburg. After years of resistance by the B&M, construction on North Union Station began in 1893. The station was built as an eastward expansion of the B&L station, with a total frontage of 568 feet (173 m) on Causeway Street. The center of the new facade was an 80-foot (24 m)-high granite triumphal arch flanked by four massive columns. The east side was formed by a five-story baggage and express building. The station was designed by the firm of Shepley, Rutan and Coolidge, which designed South Station several years later.
The new station was opened in stages from August 1893 to June 1894. The Eastern depot had been demolished in 1893 to allow construction to proceed. The B&M depot was demolished in 1897, with the site used for the Canal Street incline of the Tremont Street subway. The Fitchburg was leased by the B&M in 1900, after which the former Fitchburg depot was used as the B&M offices. By that time, the station was popularly known as "North Station". The former Fitchburg depot burned on January 17, 1925; it was demolished in 1926-28.
North Station