...the southbound 
were sent down the 6th Ave line after W59-CC to W4 and then via their regular routes through the Cranberry tubes, while the 
was sent down 8th Ave until W4 and then via the IND 6th Ave line to a nearby terminal (such as 2 Ave-Houston St)? Of course if the did that and terminated north of Jay St it would have to do it in both directions.
Has anything like this involving the northbound ever happened? My main concern is with southbound service, and I am asking mainly because I want to know if there have been any situations within the last several years in which a person wanting to go from an IND 8th Ave line station b/w W59-CC and W4 (not inclusive) to an IND Fulton line station has had to take a train to W4 upper and then walk down to W4 lower.
This has to do with figuring out which cars to be in, in order to be right next to stairs leading to the other level of platforms (via the mezzanine) upon arriving at the station. Yesterday I figured this out for all the types of trains that are used on the IND 6th Ave line (8 car trains of R46s, R68s, R68As, and R160s, and 10 car trains of R160s), for going from W4 lower to W4 upper.