I think the N (and the R) can't run through dekalb unless is it running through the tunnel and from the local pacific street track. In order to imrpove the N service after the Manhattan Bridge was finished, they made the N super express, 38th Street, Pacific, and over the bridge to Canal Street. That bumped the Q to the Broadway line express via Atlantic Avenue, Dekalb and Canal Street, otherwise there was no Bway access from the Brighton Line. The B was then added to 6th Avenue daytime and evenings. But when there is no B train, there is no 6th Avenue Access at all from Dekalb Avenue.
I like W4th Street. This is the biggest reason why late night I end up taking a cab home from the Village.
https://www.nycsubway.org/perl/caption.pl?/img/trackmap/pm_southeast_1.png