Massachusetts House and Senate negotiators have reached agreement on a $13 billion transportation improvement bill that could pave the way for construction to begin on several high-profile projects around the state.The compromise measure is expected to go before the full Legislature later this week.The bond bill authorizes the state to borrow up to $2.3 billion for the long-stalled expansion of commuter rail to the South Coast region.

It also provides money for improvements to Boston's South Station, an extension of the Green Line to Medford, the purchase of new MBTA Red and Orange Line trains, and $300 million for local road projects.