Board members also approved the MTA's proposed method for selecting a private concessionaire to design, build, finance, operate and maintain the 25.8km line.

The BPW's approval of the PPP structure for this project means that a single private partner will be responsible for designing, constructing, operating and maintaining the line, as well as providing up to $US 900m in private financing for the $US 2.15bn project.

"The department of transportation presented a solid plan to bring private sector funding, innovation and best practices to this regionally important transit line," said  O'Malley.

The 21-station line will run inside the Capital Beltway from Bethesda in the east to New Carrollton in the west, with direct connections to Metrorail's Orange Line, Green Line and two branches of the Red Line and to the Marc commuter rail Brunswick, Camden and Penn lines.

In July the state of Maryland committed $US 280m to fund design work and land acquisition for the project. With the PPP structure approved, MTA will be able to issue a request for qualifications in the next few weeks, select the shortlist of qualified bidders by the end of the year and announce a preferred partner by autumn 2014. Construction is expected to begin in the second quarter of 2015.