The Eastern Railroad was the primary arch-rival of the Boston and Maine Railroad in the mid-1800s, until it suffered financial decline and was acquired by the B&M in the late 1880s. Completed in the early 1840s, the Eastern RR ran right along the seacoast between Boston, MA, and Portland, ME, serving such cities and towns as Salem and Newburyport, MA, and Seabrook, Hampton, and Portsmouth, NH. Crossing the Piscataqua River into Maine, the Eastern RR served among other places Kittery, Eliot, South and North Berwick, Wells, Kennebunk, Biddeford, Saco, and South Portland, ME.

After the Eastern's acquisition by the B&M, it became the B&M's Eastern Division, in distinction to the B&M's Western Division, its previous (and still current) main line a little farther inland. Sometime around World War I, they both became the Western and Eastern Routes of the B&M's new Portland Division.

Except for industrial leads in North Berwick-Wells and Saco, the portion of the Eastern Route between North Berwick and South Portland was abandoned in 1944; the portion of this division between Kittery Junction and North Berwick was abandoned in 1952, which in time meant that except for the Saco Industrial Lead, the Eastern Route was gone from Maine before 1990. The Eastern Route's abandonments in New Hampshire are not as clear, because some of the trackage was long in the "out of service" column. The New Hampshire seacoast is relatively short, and so is the length of the B&M's old Eastern Route therein. The line south of Hampton has been abandoned for many years, as has the bridge spanning the Merrimack River at Newburyport, MA (this bridge and the line north to Seabrook, NH, in fact since the 1960s). There were a customer or two in Hampton until near the end of the first decade of the 21st century, accessed by a long stub line south out of Portsmouth, but both customers disappeared within the past years, and the old Eastern Route line between Portsmouth and Hampton is now out of service, and in fact may be abandoned.

Portsmouth, NH, on the coast has been accessed for many years only from the old Western Route, via Rockingham Junction eastward on the track of the old Portsmouth Branch. The freight yard at Portsmouth is little used today, so essentially carloads are brought in for industries located in Newington, on the south bank of the Piscataqua River. Very rarely, a car or two or three are taken across the Piscataqua River to Kittery Junction, then east along the track of the otherwise long-abandoned York Harbor & Beach Railroad (a subsidiary of the B&M; most of it was abandoned during the 1920s) to the Portsmouth Naval Shipyard in Kittery, Maine.

In Massachusetts, the former Eastern Route track north to Newburyport was long out of service, but within the past dozen years or so, the Massachusetts Bay Transportation Authority (MBTA) has reconnected Boston to Newburyport. The old swing bridge spanning the Merrimack River there has been out of service since the 1960s, and given the cost to rehab or replace, will undoubtedly remain so into the future. Thus, despite discussions to get service to Portsmouth, NH, commuter service presently ends at Newburyport, MA.

See also The Saugus Branch Railroad.