In 1945 the Boston Elevated Railway, which would become part of the MTA and later the MBTA, was thinking about possible expansions of the rapid transit system. All expansions were either to use existing steam railroad tracks or be parallel to those railroads in the same right of way. They created two maps showing possibilities (note that the colors used for the lines are not the ones we're used to now; those were chosen in 1965):

These show the following extensions, in order of construction, using the modern names:

• purple: Extend the green line west from kenmore to riverside and needham (Built to riverside as the D line: 1959. Needham not planned.)
• yellow: Extend the red line south from savin hill to braintree (Built as the south shore line and braintree extension: 1971, 1980)
• red: Extend the orange line from sullivan square to reading (Built only to oak grove, with rerouting, from haymarket as the haymarket north extension: 1977)
• yellow: Extend the red line northwest from harvard to arlington heights using a southerly routing to alewife. (Built with a northerly routing via porter and davis only as far as alewife as the northwest extension: 1985)
• blue: Extend the green line north to from lechmere to woburn. (Planned only to college av or route 16 with a spur to union square: scheduled to complete in 2015)
• green: Extend the blue line from maverick to lynn. (Built only to wonderland: 1954. Possible plans for an extension to lynn in the 2020s.)
• red: Extend the orange line south from forest hills to dedham (not planned)
• blue: Extend the green line west from boyleston to riverside in a northerly routing along different tracks, where the pike is now. (Not planned)

The MBTA's current plans for expansion are described in the December 2009 Program for Mass Transportation chapter 6. Their rapid transit expansion possibilities are (ignoring the silver line which is barely more than a bus):

• red-blue connector: extend the blue line from bowdoin to meet the red line at charles/mgh (legally mandated)
• blue line extension from wonderland to lynn
• green line extension to medford hillside and union square (legally mandated)
• red line extension to arlington and lexington under the minuteman commuter bike path
• green line extension from newton highlands (D line) to needham junction
• orange line extension from forest hills to route 128

The extensions considered on the 1945 map that are no longer under consideration appear to be:

• orange line from oak grove to reading.
• green line from medford to woburn
• green line from boyleston to riverside along the worcester commuter rail line