The MTA has posted a service alert which pertains to the , , , & trains. The service alert reads:
Due to emergency track at the 81st Street Station, Inwood-bound trains and Norwood-bound trains are running local between the 59th Street-Columbus Circle Station and the 145th Street Station.
Please expect delays in service on the , , and trains at this time.
I am assuming a lack of & trains are running if the & are forced to run local.