Massport has agreed to acquire a 4-acre parcel in South Boston for a potential relocation of the U.S. Postal Service’s 14-acre postal sorting facility to make way for the expansion of South Station.

The authority would combine the parcel at 525 E St. with a neighboring 12-acre parcel it already owns as a potential location for a new postal sorting facility, the Boston Herald reported.

Relocation of the postal annex on Dorchester Avenue is needed to make way for expansion of the rail hub with seven additional commuter rail tracks. That project is under state and federal environmental review.

Massport has been involved with talks with the U.S. Army Corps of Engineers in recent years for acquisition of the E Street parcel. Massport would build a maintenance facility for the Army Corps at Devens as part of the deal to acquire the South Boston property, the Herald reported.

Plans for the South Station expansion project have narrowed down potential sites for a midday layover facility needed to accommodate the additional rail traffic: the 22-acre Beacon Park yard site in Allston, the 11-acre Boston transportation department two lot on Frontage Road and the MBTA’s 17-acre existing Readville layover facility.

The midday layover facility would not be needed if state officials opt for the North-South rail link, building a tunnel between North and South stations, which is currently the focus of a $1.5 million feasibility study.