Generally all the variable channels on a single unit, like the classic ZW, are already phased properly internally.
(Things get harry when you have multiple taps that are not common, which Lionel has done many times over in some of their classic transformers, so reference this list for the popular permutations that Olsen's kindly hosts: http://www.olsenstoy.com/cd/transfmr/ps6.pdf )
