Narig had the map with the answer, but the answer was never stated.
The junction of the Queensbridge line with the 2nd Avenue elevated was at 57th Street station at 2nd Avenue, and a train with "Conections for the north only" would very obviously be headed to 2nd Avenue and 57th Street.
The 2nd Avenue service on the Astoria and Flushing lines in the evening, with the 2nd Avenue trains going only to Corona -Willets Point Blvd (Wolds Fair) was by shuttles from 57th Street and 2nd Avenue, with crossovers south of the station to the express track which was used as the reversing pocket.
The only express service on the 2nd Avenue line in anyone's memory when I was a youngster was only during rush hours, and there was no night service. The last shuttles from Willets Point and Astoria left their terminals after the shutdown had begun, with the last trains already gone from 129th to South Ferry. They operated to 57th and 2nd to pick up the passengers from the last trains from City Hall and South Ferry. And my parents and I could have ridden over the Queensboro Bridge on an el train, which I never did, and then changed at 57th for a northbound 86th and then ride the 86th Xtown bus (M18, even today!) home. But we didn't and we simply rode the IRT to Times Square and the B'way 7th Avenue local up to 86th.
And all 2nd Avenue service over the Bridge was by gate cars. I was told MUDC's the wood cars the IRT enclosed with enclosed vestibules and mu sliding doors operated by one conductor, were never used in Queens IRT elevated service.
Chicago, as well as older NY fans should remember the music at each station as the rear conductor between the last and next-to-last cars, pulled the bell rope (one in each direction on each car) to sound the bell under the roof of the front platform of the forward car, then the signal was past up to the platform in front of the motorman's window, as each conductor forwarded the message. No two bells were exactly the same pitch and harmonic structure, so one got dink dink - dang dang - donk donk - ding ding - gong gong. Ah, the days of open platform gate-car trains!
Whoever mentioned the Green Hornet? I was clear this was an IRT service, and the experimentals were BMT cars. Too wide for IRT clearances beyond Queensboro Plaza anyway! That's why the BMT used 1300 series composite gate cars in the service to 1938, then the Q's rebuilt from the 1000 and 1100 series wood cars.