I would hook it up to the fixed 12v or so on the transformer unless you have a modern engine that requires a constant track voltage.
If you use a lock out, using conventional control and have engines with 2 way e Units with out neutrals, you will have the train powered up and maybe moving to have power to the uncoupling track.
It is easy to hook up the transformer with one wire going from the constant 12v terminal, to the switch and then to the electromagnet on the uncoupling track. You should not need a second wire if the other wire from the electromagnet is hooked to the outside rail of the track at the uncoupling track. This is the way my 19 uncoupling track are hooked up.
Charlie
