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To help me understand what the “H Street Underpass” is for my story this week, Union Station Redevelopment Corporation president David Ball offered a tour of the gigantic space that might have temporarily housed the western end of the District’s first streetcar line, had DDOT and Amtrak been able to come to an agreement.

The tunnel—-accessible through a roll-down door in the stone wall on 1st Street NE at G Street—-runs the width of Union Station underneath Amtrak’s tracks and the Hopscotch Bridge. I didn’t have a flash bright enough to illuminate the tunnel’s more shadowy recesses, but perhaps this gives you a sense of the scale that we’re talking about.

Part of the reason it took so long for Amtrak and the District to kill the streetcar-in-the-underpass idea is that nobody’s quite sure who owns it; Ball thinks that the District originally did, and Amtrak may have obtained some rights through a lease agreement. But it’s all apocryphal, and USRC is only now undertaking a study to figure out who’s entitled to what.

In the short term, Amtrak wants the space as a staging ground for redoing the train platforms above—-currently, they’re not wide enough to be loaded on both sides. That may require moving around some of the steel pillars that hold up the tracks, but they can’t move the  concrete columns that support the entire station, because they’re huge.