Prince George’s County Executive Rushern Baker III. (Bill O'Leary/The Washington Post)

Prince George’s County has tentatively agreed to commit an additional $20 million to finance the Purple Line in exchange for assurances from state transportation officials that construction will begin within its borders and the command center be built there, a top aide to County Executive Rushern L. Baker III said Thursday.

Since late June, county leaders have wrangled with Maryland Transportation Secretary Pete K. Rahn’s office over the financing details for the 16-mile light-rail line, after Gov. Larry Hogan (R) said the state would build it only if Montgomery and Prince George’s counties contributed more money. Estimated to cost $2 billion, the Purple Line would run between Bethesda and New Carrollton.

News that the project would move forward was met with celebration in Montgomery County in June, where weeks later officials agreed to dedicate $40 million more to the project. But Prince George’s officials struck a more cautious tone, saying their original $100 million commitment was “unprecedented.” Any more money would have to be negotiated, they said.

Baker’s team, facing its own budget constraints, held out for weeks to score the concessions “rooted in large measure on the historical inequitable treatment” the county has faced in mass transit construction, according to an Aug. 7 letter to Rahn.

Under the agreement, Prince George’s will provide $120 million in bond financing and $10 million in in-kind contributions, such as county-owned land, as long as “significant construction activities” — the groundbreaking and ribbon-cutting — happen in the county. Also, the agreement states, the operations control center must be built in the Glenridge community, near New Carrollton.

“I agree to accomplish each of these requests,” Rahn replied in an Aug. 12 letter to Baker.

The details of the agreement are expected to be outlined in a memorandum of agreement to be completed next month.

Baker (D) and Rahn met Thursday to discuss the project. A formal announcement is expected Friday.

Baker has said the Purple Line is a priority for his administration and “will create jobs and spur the economy” along a route that will link Amtrak with the University of Maryland and west through the county’s International Corridor to downtown Silver Spring.

The agreement brings the project closer to fruition, but state transportation officials have a long road ahead to work out the numbers and fill gaps.

Maryland must still negotiate $900 million in federal funding and more than $670 million in private-sector financing. Hogan cut the state’s original commitment to the project by $500 million and made cost-savings cuts of up to $215 million.

State officials said they expect to select a winning bid to design, build and operate the line by Jan. 15. Barring delays, construction could start as early as May.