Already replied, but forgot...
I will also note WMATA does not need a pocket track to do a short turn run./i>
By that logic, a few potential midway rund for anything but Red (Red;s fine with Shady Grove-Glenmont/Grosvenor-Silver Spring runs... the short runs are the "gap trains", correct?
Blue: Stadium-Armory to National Airport (old school! Extend to Addison Road once Largo opens); Braddock Road to Federal Center SW
Yellow: For the sake of argument, extend Yellow to Greenbelt. Then, the midway runs would be Fort Totten to Pentagon City (The long and short Blue runs and full route Yellow runs would cover Crystal City and National Airport)
Green: Fort Totten, Georgia Ave-Petworth, or U Street-Cardozo to Anacostia or Southern Avenue
Orange: This would be tricky, because there's not really much distance from Cheverly to New Carrollton, and the Silver (and Blue if they did do the short runs) Line would be keeping Stad-Arm's pocket track busy, so you'd likely end up running trains from New Carrollton to Ballston or East/West Falls Church (I forget if it's Court House or Clarendon that as the other crossover)
Add in potential hybrid mixing of these four, and we could see some interesting sights:
New Carrollton to Franconia-Springfield
Greenbelt to Franconia-Springfield
Greenbelt to Huntington (wait... never mind that one)
Largo to Vienna
So picture the potential tunrbacks mid-line...
Addison Road to West Falls Church
Fort Totten to Van Dorn Street
Cheverly to National Airport (been done, I know...)
Van Dorn Street to Mt Vernon Sq-7th Street
... and anything else possible.
Suddenly, the Red Line looks rather boring by comparison... though it has some potential for multiple turnbacks as well. (Twinbrook-Takoma. Forest Glen-Friendship Heights. Rhode Island Ave-Van Ness-UDC)