There should be enough for 4 Av if the MTA forces people to go via Coney Island for the West End/Sea Beach lines.
No chance in the world of busing Queens Blvd.
PATCO, PATH, and SIR operate on 30 to 60 minute headways at night. Because their systems are small, they can single track without significant delays.
I don't know how CTA does it.
Single tracking won't work in NYC, just look at WMATA where passengers are told to expect 30-40 minutes of extra travel time.