The federal government approved Maryland’s proposed Purple Line for detailed engineering Friday, moving the 16-mile light-rail line a significant step forward in its decades-long trek toward construction.

The action by the Federal Transit Administration means the state can fine-tune cost estimates and construction schedules and complete environmental studies, officials said.

“We’ve done a lot of work so far, but it’s all been conceptual,” said Henry Kay, who oversees projects for the Maryland Transit Administration. “This allows us to hone in on how the project will be built.”

The east-west rail line, which would link Bethesda in Montgomery County to New Carrollton in Prince George’s County, has been on the Montgomery master plan for more than 20 years. The Purple Line, which would feature 21 stops, would travel mostly along surface streets and be powered by overhead wires. It would also link both branches of Metro’s Red Line at Bethesda and Silver Spring, the Green Line at College Park and the Orange Line at New Carrollton.

Officials are also looking to the transit project to help stimulate redevelopment in some of the aging communities in the Washington suburbs.

If other federal approvals and funding are granted for the Purple Line, state officials estimate that construction would begin on the Purple Line in 2015 and service would begin in 2020.

“This action by the Federal Transit Administration will help us expand rapid and reliable transportation in the Washington suburban region as part of our larger effort to create the next generation of transit in Maryland,” Gov. Martin O’Malley (D) said in a statement. “Today’s federal approval affirms the benefits and feasibility of this project — paying the way for job creation and future federal funding.”

The preliminary engineering phase will take two years and cost $46 million, Kay said. Once it is completed, federal officials will have to approve the start of the next phase, known as final design engineering.

The federal government grants state transit planners permission to proceed from one stage of planning and design to the next as part of a multi-year “New Starts” competition for federal construction money.

Kay said the approval of the engineering does not mean the project has funding. But Kay was pleased that the approval also came with a “medium high” ranking in the “New Starts” competition from the FTA.

“They’ve given us positive feedback,” Kay said. “A medium high is among the best in the country.”

The entire Purple Line project is projected to cost $1.93 billion. Half of the money for construction costs — $962 million — is supposed to come from the federal government. The other half would come from the state.

But the state and federal governments haven’t budgeted for construction of the project. Both have been grappling with shortages in transportation funding. Last month, the Maryland Transportation Authority approved statewide toll increases to offset declining revenue and to pay off debt. Maryland has deferred hundreds of millions of dollars in transportation projects in recent years because of a lack of money.

The Purple Line also must compete with transit projects around the country for federal money. Moreover, it faces opposition from people who say it will destroy the wooded Georgetown Branch trail between Bethesda and Silver Spring.

Friday’s approval comes just months after the federal government gave the state permission to move ahead with engineering on a 14.5-mile Red Line in Baltimore. Both proposals are projects of the Maryland Transit Administration and would need funding.

But officials from around the region still said Friday’s approval was a critical step in the process.

“Moving the Purple Line into this next stage is an important step towards finally getting shovels in the ground and workers on the job so that thousands in Prince George’s and Montgomery [counties], particularly federal employees, can stop wasting time, money and gas in gridlock,” said Sen. Benjamin L. Cardin (D) in a statement.