Brian,
I finally managed to get my hands on Digitrax DN163A0 decoders for the Atlas RS11s. They are the correct model. Other options include the TCS AMD1 and the NCE N12A0. The Digitrax is a 6 function decoder, the others only 2 (the headlights). I’m not sure that the TCS is available yet. The DN163A0 is a little hard to find, but Tony’s has them.
Installation is not tough, but there is a lot that can go wrong. The notches in the loco frame are exactly large enough for the decoder. The result is that the instructions underestimate how much you need to loosen the screws to separate the frame. The instructions say to loosen them just enough to remove the lightboard and squeeze in the decoder. My advice is to forget about that and remove the half of the frame the motor is not attached to in order to make removing the lightboard and installing the decoder easier. It is much easier to seat the decoder in the slots on half the frame, then reattach the other half, lining up the slots and the decoder when you do it. Just be careful not to lose nylon insulating washers that sit between the halves of the frame.
Otherwise, the brass contact strips that run from the motor to the bottom of the decoder can move slightly and miss the contacts on the decoder. If the lights come on but there is no sign of life in the motor, that is the problem. I had the same problem with an Atlas SD7. It is an easy fix that can be done with a small screwdriver while the loco is assembled. Just nudge the brass contact strip slightly forward.
Other potential problems include the fact that the contact strips can slip up during reassembly and wind up above the decoder. That one will required removing the decoder and reseating it, with the contact strips underneath. The motor can also twist slightly in its housing in the whole process, which will cause it to bind up and not turn. If that happens, loosen the frame halves and straighten the motor to seat it properly.
O