Around 1965 or so, due to a water main break, the 6th Ave. line was closed between W. 4th St. and 34th St. for a week or more. (This was pre-Christie St.) During this period, the F train terminated at 34th St., the BB ran all day (168 St/Wash Hts to 34th St/6th Ave), the D train was retagged DD and ran local from 205th St. to Coney Island via 8th Ave., Houston St., and the Culver line. There was also a C train which ran rush hours express from 205 St - I don't remember where the south terminal was (maybe Hudson Terminal, now WTC).
-- Ed Sachs