the NB trackways swung further away, but the northbound tunnel is not only still there, it's how one gets to Grand Central Tower.
I linked to the timestamp. This is the point where the Northbound track leaves the original tunnel. The current NB local ramp was dug out beneath the original NB local. behind that white door is the remaining northbound tunnel. that wall is actually relatively new, like, late 90s-ish. When I first started going on transit museum tours with my grandfather in the mid 90s, that wall wasn't there and the NB 6 would just be barreling at us until it dove down and to the side.