I think at rush hours the Q often stops at 49th. Since there's no local/express crossover between 49th and 42nd, if an express train stops at 49th it has to use the local track at 42nd also.
Here is a track map of midtown.
The N expresses I've seen all use the express tracks from 14th to 57th and terminate at 57th. I don't know about below 14th because when I ride the Broadway BMT in the morning I'm getting on at 14th.