In 1907, the lease of the Brooklyn Union Elevated Railroad was canceled, and this company began to operate most of the rapid transit lines.
A new company, the New York Consolidated Railroad, was formed in 1912 as the rapid transit operating subsidiary, and that same year the New York Municipal Railway was formed to enter into Contract 4 of the Dual Contracts with the city, under which the BRT gained subways and elevated extensions.
The Sea Beach Railway (Sea Beach Line) and South Brooklyn Railway (Culver Line), which had been operated by the Brooklyn Heights as part of its elevated system,[citation needed] were released for independent operation.
Since then, many changes have been made; see the individual articles about the letters for more detail, and New York City Subway nomenclature for more general information.
While the BMT number code was officially retired in the 1960s, two of these markers would continue to appear on equipment until as late as the early 1980s.
For instance the BMT 7 marker appeared on Franklin Avenue Shuttle trains as late as 1982.
When BMT service began on the Corona and Astoria Lines in 1923, a Queens Division was added.
There were some instances of joint trackage, mainly on the Nassau Street Line and approaching Brooklyn Bridge.
The remaining Brooklyn lines are on elevated structures, in open cuts or on embankments, or on short portions of surface trackage.