A new wrinkle could delay the start of service on Metro's new Silver Line: the completion of a rail yard where trains will be stored and repaired.

Metro officials said Thursday that the scheduled completion date for the West Falls Church rail yard does not give them enough time to train their workers and have trains begin offering service in late 2013 as promised.

The Metropolitan Washington Airports Authority is building the first 11 miles of the new rail line that will extend from near East Falls Church to Tysons and eventually beyond. It plans to finish building the rail line to Metro's specifications, then hand it over to Metro in August 2013 for the agency to begin testing and training before the line opens to riders.

But Metro said Thursday that the yard isn't slated to be finished until Dec. 20. Agency officials would not pinpoint how much time they needed to prepare, test and train workers on how to use the rail yard before service could begin running on the newest Metrorail line.

"We will do all the testing in places we can, but at some point we need that yard," General Manager Richard Sarles told reporters.

He said Metro has been talking to MWAA about the problem and what it means for the Silver Line's start. "They understand the situation and they are doing what they can to advance the schedule," he said.

MWAA spokesman Rob Yingling said he was not aware of anything changing the target for when MWAA would hand over the rail line. But a September MWAA report suggests the rail yard completion could be even later than Metro stated.

The report detailing progress made as of July 2012 forecasted that the yard would not be finished until Jan. 30, 2014, which it noted was 41 days behind the "agreed target date" of Dec. 20, 2013. Project officials blamed the delay on a subcontractor and equipment failures during tests.

The news about the possible delay came as Metro detailed new service plans for the line. The agency said trains will run to Largo Town Center, instead of stopping at Stadium-Armory because there is not enough space to safely change the trains' direction at the Blue and Orange line junction. The pocket track there is 602 feet long and the eight-car trains slated to run on the Silver Line are 600 feet long, Deputy General Manager Dave Kubicek said.

There is also minimal room on the sides for workers to reach the aerial structure. Retrofitting the existing pocket track would cost at least $300 million, if it could be pulled off, Sarles said.

The extension to Largo will cost an estimated $1.5 million more in the first year for operators and energy costs, officials estimated, part of $4.6 million in additional Silver Line operating costs.

Staff writer Liz Essley contributed to this report.