Wow... well, at least it clears up what I'd thought... it's quite true, the majority of crossovers were likely intended for emergency single-track ops.
Why the recent turnbacks at Dupont Circle? And when do the Red trains turn back at Farragut North? I thought the last time anything did that was when the Green Line Shortcut was still operational (then again, from what I've heard, WFC and National Airport occasionally have Orange or Blue (but never Yellow) trains terminate there).
In fact, the ONLY turnbacks I've heard of being used regularly are the current batch s it stands today: End-to end on the shared four (if you count Mt Vernon Sq-7th Street as the Yellow Line's "end"); Grosvenor, Shady Grove, Silver Spring, and Glenmont on Red. Other possible terminal stations are only used to short-originate trains so they arrive downtown by the time of normal opening of the system for the day (so you're not waiting 30+ minutes for a train in downtown DC).