The estimated cost of building a light-rail Purple Line through the Maryland suburbs has risen an additional $56 million, bringing the total projected budget to $2.43 billion, according to a federal report.

The latest estimate is contained in an online Federal Transit Administration report from July, but wasn’t publicized, and it reflects the project’s second cost increase in seven months. An FTA report from January said the Maryland Transit Administration had increased cost projections by $220 million to reflect finance charges and escalating real-estate-acquisition costs. The pre-July cost estimate was $2.37 billion.

The Maryland Transit Administration updates the FTA on project cost estimates as it competes for federal aid. Maryland is seeking $900 million in federal money to build the 16-mile east-west light-rail link between Prince George’s and Montgomery counties.

Henry Kay, who heads transit project development for the state, said the MTA didn’t publicize the increase because it was considered a “minor adjustment” on such a large, complex project. He said the additional costs came from refined estimates based on more detailed engineering and still-rising real estate prices.

“It doesn’t reflect some faulty approach” to cost estimating, Kay said. “It’s just the nature of a mega-project being developed over a number of years.”

Word of the Purple Line’s new price tag follows news that the estimated cost for a proposed 14-mile light-rail Red Line in Baltimore had ballooned by $267 million, to $2.9 billion. State officials attributed that increase to unexpected challenges discovered during engineering, including the need to dig a deeper tunnel beneath downtown and build a station amid surprisingly high underground water pressure, according to the Baltimore Sun.

A Purple Line would have 21 stations and run two-car trains between Bethesda and New Carrollton, along local streets and the wooded Georgetown Branch Trail. It is designed to link Maryland’s spokes of the Metrorail system with Amtrak and with MARC commuter rail stations, in addition to providing a faster and more reliable transit option than buses.

The $56 million increase did not affect the project’s overall federal rating of “medium-high.” The rating helps determine which projects nationwide receive highly competitive federal construction funds.

Even so, the increase took local Purple Line watchers by surprise, including transit advocates who have worked closely with state officials for years. Some pointed out that the true cost of the project won’t be known until the state awards a contract for a 35-year public-private partnership, in which a team of private companies would help finance the line’s construction, in addition to designing, building, operating and maintaining the line long-term. Bids are due Jan. 9.

“This is a design-phase cost estimate,” said Ralph Bennett, president of Purple Line Now. “I do wish they’d let us know, but we don’t really know what it’s going to cost until a private partner is chosen.”

But others said they are increasingly concerned as costs mount.

“We’ve been pretty nervous about the fact that the numbers keep going up,” said Joan Fidler, president of the Montgomery County Taxpayers League.

The additional $56 million would come from the state’s transportation trust fund, bringing the fund’s contribution for Purple Line construction to $726 million, according to the FTA report. The fund also pays for state highway, aviation and other transit projects.

State Sen. Richard S. Madaleno (D-Montgomery), who has scrutinized the state’s Purple Line financial plans, said dipping into the fund to pay for two major transit projects whose costs continue to escalate could erode political support.

“People might say why is my local interchange not being built because of big transit projects in [the] Washington [suburbs] and Baltimore?” Madaleno said.

MTA officials have said they hope to begin construction on a Purple Line in mid-2015 and open it in 2020.

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