The federal government has approved Maryland’s proposed Purple Line for more detailed engineering.

The go-ahead is a necessary step in moving the project forward and means officials can fine-tune cost estimates and construction schedules and finalize environmental studies, state officials said.

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.

The 16-mile line would originate in Bethesda and travel through Silver Spring and College Park to the New Carrollton transit hub in Prince George’s County. The light rail line, which would feature 21 stops, would travel mostly along surface streets and be powered by overhead wires.

“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.

The state’s financial plan assumes that the federal government will cover half — or $962 million — of the construction costs, but the question of how to pay for the project remains.

Maryland has deferred hundreds of millions of dollars in transportation projects in recent years because of a lack of money. The state also recently decided to raise tolls statewide to help confront its deteriorating transportation infrastructure and a paucity of funds to make improvements. However, officials said in their announcement Friday that construction on the Purple Line could start in 2015 with service beginning in 2020.

“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 County, particularly federal employees, can stop wasting time, money and gas in gridlock,” said Sen. Benjamin L. Cardin (D).

This summer the federal government also gave Maryland permission to move ahead with engineering of a 14.5-mile Red Line in Baltimore, which would cost $2.2 billion. Both proposals are projects of the Maryland Transit Administration.

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Complete coverage: The Purple Line