The R68 cars are currently the exclusive rolling stock for the
B,
D, and
Q Trains, as well as the
Franklin Avenue Shuttle and form most of the rolling stock on the
N line. They do not currently run on the
R.
R68s were the second generation of 75' cars (after the R44/46 orders of the early 70s). Unlike the R44/46s, the R68s are disgusting looking and poor performing. They feature mirrored interiors which amplify every bit of grimyness, and space reducing narrow cabs at all car ends, as opposed to the R44/46s which have cabs only in the conductor and motorman's cars.
One has to wonder what the MTA was thinking.