sptrains;
I know you do this for a living... so do I, in a smaller way. But I am also a teacher, and one thing you learn in teaching is that the group has to be clear on terms and semantics. I don't want to sound like a know-it-all, but please indulge me while I give my understanding of terms.
A "transformer" is an electrical device that uses electromagnetism to induce AC current in one coil from another, and in the process the transformer changes the voltage level. Three-rail systems use AC transformers because their locos are built to run on AC current.
A DC "power pack" is a device that converts AC current to DC. It may use a transformer and a full wave bridge rectifier, but modern ones may be entirely solid state (using silicon "chips"). Two rail systems are all DC (unless someone's doing something I haven't heard of yet).
An excellent tutorial on DCC and its terminology is found on the
Armadillo and Western website. Its diagram does show a separate power supply for the command station (which I frequently call a controller by mistake), and the booster.
Super Empire Builder includes the
DB150 Command Station/Booster combination, which I now see requires an external power supply. Because the input to the DB150 may be either AC or DC you can use either a transformer or a power pack. I learn something new every day.
I find this very curious because the Zephyr does not require an external supply. I guess this is because the Zephyr is intended for use with HO or smaller scales, and the DB150 can drive scales up to G depending on the attached supply. This definitely makes the Zephyr a far better buy unless you are building a larger scale layout, or one which will require more than 2.5 amps, because a good power supply will easily run $50 or more and a cheap one at least 25. I just had a friend of mine pick up a dual supply kit for his kids to build (we're teaching them basic electronics). This dual 15vdc supply kit ran $65. The beauty of it is you can put your booster on one supply output and your accessories on the other.
Thanks for the input. I appreciate your posts.