Metro plans to rely on its existing rail fleet when the planned Silver Line opens because of delays with the production of new rail cars and a new facility designed to test them.

That could strain the system's existing service, maxing out the agency's already tired fleet of 1,120 cars.

Metro has awarded a $66 million contract to Skanska to build a test track facility near Metro's Greenbelt rail yard, the Swedish company said Monday. The facility will not be finished until March 2014, according to Skanska.

But that's several months after the first leg of the Silver Line is supposed to open, scheduled for December 2013 or early 2014.

The contract requires Skanska to let Metro begin to use the test tracks in January 2014, while finishing touches are made, Metro spokesman Dan Stessel said. Even so, it can take several weeks to test each new rail car, so there will be a gap between the opening of the line and testing cars on the new track. That means Metro will have to test at least some new rail cars on existing tracks, Stessel said.

But production delays on the new cars because of the March 2011 tsunami and earthquake in Japan, plus other issues, means that only a "handful" of the new rail cars will arrive in time for the planned opening of the Silver Line, he said.

Metro ordered 428 rail cars from Kawasaki in 2010, 300 of them to replace the 1000 Series that have been called death traps and 128 of them for the planned Silver Line. The agency wanted to have 64 rail cars for the first phase of the Silver Line, now under construction.

But before any new rail cars can carry riders, they must be run a set distance along actual Metro tracks.

In the past, Metro has run the cars on its tracks during closed hours. But back then, the agency wasn't using midday, late night and weekend hours to squeeze in track work.

And this is the largest order of rail cars in the agency's history. Metro has said for years that it needs a separate set of tracks to test so many trains.

However, Stessel said, he doesn't anticipate that initial testing would disrupt track work. The test track should be ready for most of the new cars.

But, he said, it's a "difficult question to answer this far out," about whether using other rail cars from the other lines would degrade service.

He said the agency knows how important having eight-car trains and its new Rush Plus service will be. "We're going to make every effort to make sure there's adequate capacity."