The Boston Subdivision is a railroad line owned by CSX Transportation in the U.S. state of Massachusetts. The line runs from Boston west to near Springfield along a former New York Central Railroad line. Its east end is at Amtrak's Northeast Corridor (this portion owned by the MBTA) at Back Bay Station, over which CSX has trackage rights to the east to reach the Dorchester Branch. Its west end is in Wilbraham, east of Springfield, at the east end of the Berkshire Subdivision. Along the way, the line junctions the Framingham Subdivision and Fitchburg Subdivision at Framingham.
The line east of Riverside is owned by the Massachusetts Turnpike Authority; from Riverside west to Framingham, the Massachusetts Bay Transportation Authority (MBTA) owns the line. MBTA Commuter Rail Framingham/Worcester Line trains operate over the line east of Worcester. The line originally had four tracks to Riverside Station (two curved to the south and are currently used by the MBTA's Green Line's D Branch light rail service. The number of tracks running into downtown Boston was reduced to two, in order to build the Massachusetts Turnpike, which parallels the easternmost ten miles of trackage, although CSX retains the original Boston and Albany Railroad trackage rights.
The Boston and Worcester Railroad opened its line from Boston west to Worcester in 1834 and 1835. The Western Railroad opened from Worcester west to Springfield in 1839. The line became part of the Boston and Albany Railroad, New York Central, and Conrail through leases, mergers, and takeovers, and was assigned to CSX in the 1999 breakup of Conrail.