WASHINGTON – The Metropolitan Washington Airports Authority is investigating why a Silver Line test train derailed Tuesday.

The train’s front axle popped off the track when it hit a derailer west of Wolf Trap, Pat Nowakowski, executive director of the Dulles Rail Project, tells The Washington Post.

A derailer is a mechanism that will derail a subway train if it goes farther on a track than it is supposed to go.

Test trains started running on the Silver Line tracks late last year, but Nowakowski says this it the first time there has been a derailment during the runs.

The first phase of the Silver Line will go out to Wiehle Avenue in Reston and is expected to open early next year.

The line eventually will take the subway from East Falls Church through Tysons Corner, Reston, Herndon and out to Dulles International Airport and Ashburn.

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