The Eastern Railroad was a railroad connecting Boston, Massachusetts to Portland, Maine. Throughout its history, it competed with the Boston and Maine Railroad for service between the two cities, until the Boston & Maine put an end to the competition by leasing the Eastern in December 1884. Much of the railroad's main line in Massachusetts is used by the MBTA's Newburyport/Rockport commuter rail line, and some unused parts of its right-of-way have been converted to rail trails.

The Eastern Railroad Company of Massachusetts was first chartered on April 14, 1836. The line followed the coastline, in contrast to the Boston & Maine's inland route through Massachusetts, and it served North Shore cities such as Lynn, Salem, Beverly, and Newburyport. In keeping with its coastal route, the Eastern Railroad chose to place its Boston terminus in East Boston, a short ferry ride from downtown Boston, rather than building tracks around Chelsea Creek, the Boston Inner Harbor, and the Mystic River into the city.

Construction on the railroad began in August 1837 after state loans and a change of route were approved in April. The first stretch to be built was from East Boston to Salem (13 mi or 21 km), completed August 27, 1838. An extension to Ipswich (12 mi or 19 km) was completed on December 18, 1839, followed by an extension to Newburyport (9 mi or 14 km) on August 28, 1840, and to the New Hampshire state line (10 mi or 16 km) on November 9, 1840. A branch line to Marblehead opened on December 10, 1839, followed by a branch line to Gloucester in 1847 and a branch line to Amesbury in 1848. In 1861, the Gloucester branch was extended to Rockport. On August 31, 1846, the Eastern leased the Essex Branch Railroad for 5 years, and in 1865 it bought the branch outright.

The railroad's short segment through New Hampshire was chartered as a separate corporation by the New Hampshire legislature on June 18, 1836. Construction on the New Hampshire segment began in 1839 and was completed on November 9, 1840. On February 18, 1840 the Eastern Railroad of New Hampshire was leased to the Eastern Railroad of Massachusetts for a period of 99 years.

By 1843, the Eastern entered into an agreement with the Boston & Maine to share the Portland, Saco and Portsmouth Railroad's tracks in Maine, which allowed both railroads to begin providing Boston-to-Portland service. On April 28, 1847, the Eastern and the Boston & Maine co-leased the Portland, Saco & Portsmouth for a period of 99 years.

By the 1850s, the Eastern was experiencing difficulties because of the out-of-the-way location of its East Boston terminal. In 1845, the competing Boston & Maine Railroad had completed its own tracks into Boston so it would no longer have to use the Boston and Lowell Railroad's tracks. It also built a terminal in downtown Boston just north of Haymarket.

Several independent railroads sought to take advantage of the situation by building branch lines that would connect the Eastern Railroad's North Shore tracks with the Boston & Maine line going into the city. In 1850, the South Reading Branch Railroad opened, connecting the Eastern at Salem to the Boston & Maine at Wakefield, and in 1853, the Saugus Branch Railroad opened, connecting the Eastern at Lynn to the Boston & Maine at Malden. The Eastern bought the South Reading Branch Railroad in 1851 and the Saugus Branch Railroad in 1866.

The Eastern Railroad was finally able to offer service to downtown Boston when it leased the Grand Junction Railroad in 1852. The Grand Junction was a short line chartered in 1847 that connected the East Boston waterfront to the Boston & Maine, Boston & Lowell, and Fitchburg railroads in East Somerville, and it was eventually extended to connect to the Boston and Worcester Railroad in Allston. After leasing the Grand Junction, the Eastern built a cut-off from the Grand Junction to its own tracks in Chelsea and built a terminal in downtown Boston, approximately on the site of the present North Station. It also disconnected the Saugus Branch from the Boston & Maine at Medford, redirecting it south to the Grand Junction in Everett. In 1866, the Boston & Worcester bought the Grand Junction, but allowed the Eastern to keep its track rights for the sections it used as part of its main line.