If you want to use the original Athearn motors, you could try something like the Digitrax DH123-AT, which is specifically intended for such applications.  The instructions are pretty clear so you should have no trouble adding decoders using them.  However, you'd still be stuck with performance issues inherent with the Athearn motors.

A-Line has a can motor kit (#90321) for the Athearn GP38-2, 40-2 and 50, see http://ppw-aline.com/re-power.htm.  The version with the pre-formed cradle weight may be easier to install, #12605. 

Any "hard wire" decoder will work fine with the A-line motor, like the Digitrax DH165-AO or NCE's DA-SR.  Again, follow the instructions on motor isolation and you should be fine.  Most of my locos are older mechanisms equipped with such decoders and while they take some time they aren't too tough. 

I don't use sound myself, but there are hard-wire sound decoders that should work well also, and installation around the motor is still the same.

You may also want to look into additional tune-up and wiring to ensure reliability, like the "5-wire" procedures documented in various places like http://www.mcor-nmra.org/Publications/Articles/Athearn_TuneUp.html.

Thanks for all the ideas. I stopped by my LHS this evening and he sold me the DHAT and the Tsunami Tsu1000. It is the newest direct replacement for the older RTR Athern locos. The neat thing is it is solderless and with the Tsunami, all you have to do is cut the shrink wrap where the wires connect and plug the digitrax DHAT directly into the decoder. You get DCC and sound without all the fuss... so its supposed to go. I will let you know how it goes.

Follow up and let us all know how it works out.

Hi Cliff,

Don't know if this will help at all, but I documented my Athearn GP38-2 remotor and sound install at http://www.iaisrailfans.org/gallery/Upgrading-Athearn-IAIS-GP38-2s ,

Hey, I'm coming a bit late to this, but I used a Digitrax DH123-AT in an older Athearn GP38-2 that I picked up cheaply at a recent swap. I'm sort of new to DCC and this was my first decoder install but the instructions are very clear and installing the decoder was pretty straightforward. Toughest part was making the wires neat so they wouldn't catch on the shell when it goes back on. 

I wouldn't waste time buying one. Athearn say they are no longer offering decoders with these type of harnesses. See the link here on their website

The next step (for me anyway) is replacing the 1.5 V lights with LEDS. The unit came with a 1.5 V light in the cab end only but it looks awful, so I'm going to add LEDs to the cab and back end. 

Good luck!

I would not trash the old Athearn motor!  There is a wealth of information on the internet to make those units operate as good as more expensive units. Yes, they will still draw more amperage than an Atlas or Kato unit, but making the proper mods to the motor, drive train and wheels will make a HUGE difference in how they run! Check out my article in the 2011 MRH tips section on page 20 for wheel cleaning. http://issuu.com/mr-hobbyist/docs/mrh11-08-aug2011-ol?viewMode=presentation&mode=embed

Have fun and happy modeling!