Hi Will,
The tail tank on the 747-400 are optional tanks. The operator asks Boeing to fit it in or not when the plane is being build.
Since fuel on the tail is a nightmare for every loadmaster, freighter operators tend to keep their fleet without the tail tank!
By the way, all LHCAG
MD-11 fleet has a tail tank!!! Mind you, having a huge CF6-80 and 10 tonnes of fuel on the tail isn't a good idea!!!
See ya,
GHN