The B&L built or leased many branches to serve areas not on its original line. Immediately before its lease by the B&M in 1887, it had five divisions—the Southern Division (including the original line), the Northern Division, the White Mountains Division, the Vermont Division, and the Passumpsic Division. Additionally, it leased the
Central Massachusetts Railroad in 1886.
Southern Division The main part of the Southern Division was the mainline between
Boston and
Lowell. ;Charlestown The
Charlestown Branch Railroad was not itself taken over by the B&L, but as originally built in 1840 it was a short spur from the B&L to
wharves in
Charlestown. In 1845 the
Fitchburg Railroad leased it and incorporated it into their main line. ;Mystic River The Mystic River Branch served the
Mystic River waterfront on the north side of Charlestown. ;Woburn Loop The
Woburn Branch Railroad (aka the
Woburn Loop) opened in 1844, connecting
Woburn to the main line towards Boston. The Horn Pond Branch Railroad was a short freight-only branch off the Woburn Branch to
ice houses on Horn Pond. The northern loop, built in 1885, continued the line back north to the main line at North Woburn Jct. in South Wilmington. The Horn Pond branch line was abandoned in 1911, the northern loop in 1961, and the original line in 1982. ;Stoneham The
Stoneham Branch Railroad was built in 1862 to connect to
Stoneham. ;Lowell and Lawrence The
Lowell and Lawrence Railroad was chartered in 1846 to build a line between Lowell and
Lawrence, which opened in 1848. In 1858 the B&L leased the line. ;Salem and Lowell The
Salem and Lowell Railroad was chartered in 1848 as a branch from the Lowell and Lawrence at
Tewksbury Junction to the
Essex Railroad at
Peabody, along which it used
trackage rights to
Salem. The line was opened in 1850 and operated by the Lowell and Lawrence until 1858, when the B&L leased it along with the Lowell and Lawrence. ;Wilmington (Wildcat) Branch The Wilmington Branch, now known as the
Wildcat Branch, was built just west of the original
Boston and Maine Railroad alignment to connect the main line at
Wilmington to the Salem and Lowell at Wilmington Junction, providing a shorter route between Boston and Lawrence. ;Lexington and Arlington (Middlesex Central Railroad) The
Lexington and West Cambridge Railroad was chartered in 1845 and opened in 1846, connecting the
Fitchburg Railroad at West Cambridge to
Lexington, although the "West Cambridge" in the name referred to what is now the town of
Arlington. It was operated by the Fitchburg from opening, and leased to the Fitchburg from 1847 to 1859. The line was reorganized as the
Lexington and Arlington Railroad in 1868, following the renaming of Arlington. The B&L bought the line in 1870 and built a new connection to their main line at
Somerville Junction. The
Middlesex Central Railroad was chartered in 1872 and opened in 1873, extending the line from Lexington to
Concord. It was leased from completion to the B&L. An extension west to the
Nashua, Acton and Boston Railroad at
Middlesex Junction was built in 1879. ;Billerica and Bedford The
Billerica and Bedford Railroad was built in 1877 as a
narrow gauge line between the Middlesex Central at
Bedford and the B&L at
North Billerica. It was sold and abandoned in 1878, and the rails were taken to
Maine for the
Sandy River Railroad. A new
standard gauge branch was built by the B&L in 1885, mostly on the same right-of-way. ;Lowell and Nashua The Lowell and Nashua Railroad was chartered in 1836 as an extension of the B&L from Lowell north to the
New Hampshire state line. The Nashua and Lowell Railroad, chartered in 1835, would continue the line in New Hampshire to
Nashua. The two companies merged in 1838 to form a new
Nashua and Lowell Railroad, and the road opened later that year. In 1857 the B&L and N&L agreed to operate as one company from 1860, and in 1880 the B&L leased the N&L. ;Stony Brook The
Stony Brook Railroad was chartered in 1845 and opened in 1848, connecting the Nashua and Lowell at
North Chelmsford with
Ayer. The N&L leased the Stony Brook in 1848. ;Nashua to Keene The
Wilton Railroad was chartered in 1844. It opened a line from Nashua west to
Danforth's Corner in 1848, to
Milford in 1850 and to
East Wilton in 1851. Since completion it was operated by the N&L. The
Peterborough Railroad was chartered in 1866 to continue the Wilton Railroad northwest to
Greenfield, New Hampshire. In 1873 the N&L leased it; the road opened in 1874. The
Manchester and Keene Railroad was chartered in 1864 and opened in 1878, continuing the Peterborough Railroad west from Greenfield to the
Connecticut River Railroad in
Keene. In 1880 the company went bankrupt, and it was operated by the Connecticut River Railroad until 1882, when it was bought half-and-half by the B&L and the
Concord Railroad.
Other divisions ;Central Massachusetts Railroad The
Massachusetts Central Railroad was chartered in 1869 to build a line east–west across the middle of the state, between the
Boston and Albany Railroad and the
Fitchburg Railroad. The first section opened in 1881, splitting from the B&L's
Lexington and
Arlington Branch at
North Cambridge Junction, and the company was reorganized as the
Central Massachusetts Railroad in 1883. The B&L leased the line in 1886, a year before the B&M leased the B&L. ;Northern Division The
Boston, Concord and Montreal Railroad was chartered in 1844, and opened in stages from 1848 to 1853, eventually running from
Concord to
Woodsville, New Hampshire. That railroad, along with its branches, became part of the B&L Northern Division in 1884, when the B&L leased the BC&M. The
Northern Railroad was also chartered in 1844, opening in 1847 from Concord to
Lebanon, New Hampshire, and later extending to
White River Junction, Vermont. The B&L leased it in 1884 as another part of its Northern Division. The only connection between the Southern and Northern divisions was at
Hancock Junction, where the
Manchester and Keene Railroad (Southern) and
Peterborough and Hillsborough Railroad (Northern) met. In 1889 the BC&M merged with the
Concord Railroad to form the
Concord and Montreal Railroad, taking it out of B&M control until 1895, when the B&M leased the C&M. ;White Mountains Division The
White Mountains Railroad was chartered in 1848 and opened a line from
Woodsville to
Littleton, New Hampshire, in 1853. Along with extensions and branches, it was leased to the
Boston, Concord and Montreal Railroad in 1859 and consolidated into it in 1872, becoming its
White Mountains Division. In 1884 the B&L leased the BC&M and the old White Mountains Railroad became the B&L's White Mountains Division. The Northern and White Mountains Divisions were connected at Woodsville. ;Vermont Division The
Essex County Railroad (chartered 1864),
Montpelier and St. Johnsbury Railroad (chartered 1866) and
Lamoille Valley Railroad (chartered 1867) were consolidated into the
Portland and Ogdensburg Railroad in 1875 as their Vermont Division. The line was finished in 1877, and in 1880 it was reorganized as the
St. Johnsbury and Lake Champlain Railroad, which was taken over by the B&L as their Vermont Division. The line did not stay in the B&M system, and the easternmost part was leased to the
Maine Central Railroad in 1912. The White Mountains and Vermont Divisions were connected at
Scott's Mills, New Hampshire. ;Passumpsic Division The
Connecticut and Passumpsic Rivers Railroad was organized in 1846 and opened a line from
White River Junction on the
Northern Railroad to the border with
Quebec,
Canada, in 1867, junctioning the Northern and White Mountains Divisions at
Wells River and the Vermont Division at
St. Johnsbury. The
Massawippi Valley Railway, leased in 1870, continued to
Sherbrooke, Quebec, where it junctioned the
Grand Trunk Railway among others. The B&L leased the line on January 1, 1887, three months before the B&M acquired the B&L. ==Life as a B&M line==