When I posted my reply, I had not yet actually ran my milk car, a 6-9220 (but I think they're all the same). I ended up having some problems with the car and/or fastrack, so with your question in mind I put up a test track with tubular track and the old unloading track. It was having some, but different problems there. For some reason I then looked at the pickups, and they were way dirtier than I could imagine. I cleaned them, and it worked fine on the tubular track, then I took it back to the fastrack section and it worked fine there too.
I really like the fastrack for this car since the contact area for the shoes is soooo much bigger than the old tubular option.