I am respond to both here.
Except that about half of them are programmed in the codes, and as Oren (was it Oren? Might have been Mountain Mark) said, they typically don't use crossovers for turnbacks except at end terminals, because they'd need to hold the train either before entering to drop off, or before depatring to cross over, which might tie up end-to-end movements.
Like I said "Virtually ever location where there are crossovers can be used to turn trains". This does not necessarily mean that they will be used for short turn runs. (future WMATA quiz answer.) Roughly 625’ (190.5m) Beyond every crossovers and pocket track is a green sign with the white letters TB on it. This sign tells a train operator that if he stops his train at the sign the train is clear of the interlocking. At this point the operator can safely changing direction and proceed through interlocking in the opposite direction. The route will be selected by either local train control room from destination code transmitted from train by way of TWC Train to Wayside Communication or by central control.
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?
"Gap train" are defined as trains held in pocket or tail tracks at terminals to fill holes in the schedule the are created when trains are taken out of service because of some malfunction.
I will also note that some of the Red Line trains that terminate their runs of Silver Spring (B08) use the crossover on the south end of Silver Spring (B08) as opposed to using the pocket north of the platform. This kind of blows away the idea that pocket track are needed to terminate scheduled short turn runs.
John