Well, Harold....that's my question. I use a Digitrax system, and to my knowledge they have not released a decoder for these locos yet. I wonder if any of the other decoder manufacturers have one yet.
Do You have space for a tiny decoder this size: .55x.35x.12 in. Then a Zimo MX62 should fit. It provides .7amps continuous and 1.5 amps peak, high frequency and back EMF. See them here: mrsonline.net Art
I spoke to Digitrax at the Pomona, CA Train Expo this past weekend. They were a little dismayed and more than a little disgusted that Athearn did not produce these engines decoder-ready. The only suggestion they could offer was to try milling the frame to make just enough space ionside for a Z-scale decoder.