A Massport land deal in South Boston could finally relocate the U.S. post office from Dorchester Avenue, permitting the long-awaited $1B South Station Expansion project to move forward.

Massport signed a letter of intent Wednesday to acquire a four-acre plot of land near the Boston Convention and Exhibition Center from the U.S. Army Corps of Engineers. In exchange, the organization would build an 8K SF maintenance facility for the U.S. Army Reserve at the former site of Fort Devens in central Massachusetts, the Boston Herald reports. The Army-owned land at 525 E St., combined with an an adjacent 12-acre lot Massport owns, could be offered as a new site for the post office in ongoing negotiations.

Efforts have been underway for nearly 20 years to relocate the mail facility from next to New England’s busiest train station, which would allow seven new tracks to be built to account for increased ridership on commuter rail and Amtrak.

While there is growing sentiment to avoid expansion and pursue the North-South Rail Link tunnel between North Station and South Station, South Station Expansion has the support of Gov. Charlie Baker and is much further along in planning. The 1.2M SF, four-story post office building on the site would have to be demolished. Post office officials would only tell the Herald they were not involved with the land deal.

Above the tracks, the train station is already busy with development. Houston-based development giant Hines gained city approval in March to build a 690-foot tower over the station and is awaiting a final green light from the state. Blackstone has placed its 98-year leasehold interest of the station's retail and four-story office space on the market. Experts said the management deal could fetch close to $100M.