Design and construction professionals know that depending on the situation, one inch can be insignificant, or make all the difference in the world.

But when the comes to the Dulles Metrorail extension, one inch seems just enough to warrant headlines and worries of "not again."

The Washington Post reported this week that a portion of the 2,400-foot long Tysons Corner tunnel has settled by approximately one inch since it was dug using the New Austrian Tunneling Method in 2011.

Although no additional movement had been detected since the settlement was first noticed during backfilling work early last year, the Metropolitan Washington Airports Authority (MWAA), which is building the tunnel to serve the new Silver Line, authorized design-build contractor Dulles Transit Partners to conduct tests earlier this month to assess whatif anythingmight be happening, and why.

Because the tunnel lies as little as seven feet below the surface in some locations, Dulles Transit Partners  positioned 10 inches of steel fiber reinforced shotcrete with steel lattice girders every three feet during construction to give the overhead soil additional rigidity until the final 12-in thick cast-in-place, concrete liner. Each of the tunnel’s twin tubes has a finished internal diameter of 18 feet, 4 inches.

The results are being reviewed by MWAA and the Washington Metropolitan Area Transit Authority, which will assume operation of the Silver Line upon its scheduled completion later this year. They indicate that the settlement does not currently impair the movement of trains through the tunnelgood news for MWAA given that controversy has shadowed the Silver Line even since before construction of its $2.9 billion first phase began in early 2009.

They included the abandoned deep-tunneling option through Tysons Corner, which raised fears of cost overruns and threatened the project’s federal funding; concerns about the integrity of the seemingly ready-made pier foundations at the Silver Line’s split from Metro’s existing Orange Line tracks; and MWAA’s insistence of having an underground station at Washington Dulles International Airport as part of the project’s second phase, a stance that nearly cost the support of several local jurisdictions before the agency gave in and accepted an above-ground alternative.

Along the way MWAA’s management practices have repeatedly come under fire, compounding concerns about the agency’s technical and administrative ability to manage a rail transit project. And it wasn’t until the passage of Virginia’s compromise transportation funding plan last month that the Commonwealth’s $300 million contribution to Phase 2 had a revenue source.

Still, it seems that if something can go wrong with the Silver Line, it will. Though those test trains cleared tunnels’ roof or sides, they did strike a handrail and several utility boxes that MWAA says had been installed in the wrong places.

With that kind of luck, one might expect to see rabbits feet and horseshoes adorning MWAA offices these days, and officials with their fingers perpetually crossed. Yet the agency remains confident that Phase 1 of the Silver Line will be serving passengers as scheduled in December.

Best to bring heavy boots and a snow shovel to the opening ceremony—just in case.