Ask GGWash: Why are the Red Line tracks inside the CSX tracks between Brookland and Silver Spring?

Hi folks: I've got this curiosity that I wanted to run by you and see if anyone could shed light on it. I recently moved to Takoma, from Capitol Hill. In this part of the Red Line, Metro shares a right-of-way with CSX's Metropolitan Subdivision, formerly B&O's Metropolitan Branch. But the curious thing is that the CSX tracks straddle the Metro tracks. Why?

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