D.C. Metro’s $2.8-billion Silver Line extension, already beset with months of construction delays, is now grappling with more than 400 defective concrete rail ties. The 12- to 14-in.-long ties, fabricated by Rocla Concrete Tie, were found after installation to be up to a half-inch higher in the middle than on the ends, potentially affecting the rail’s vertical alignment. Lead contractor Capital Rail Constructors, a joint venture of Clark Construction Co. and Kiewit Infrastructure South Co., and the Metropolitan Washington Airports Authority, which is overseeing the project’s construction, are trying to develop a solution that keeps the project on track for substantial completion in August 2019.