Do a reset!

That's where I would start first.   Write CV 8 = 8.  If it doesn't respond to this on address 3, something is wrong. Disassemble 1 power feed at a time to eliminate those.  Did this locomotive run on DC?  And if so, will it run with a DC dummy plug installed?  From what I can see, it appears to be wired correctly, but I'll be honest, I do not trust those DCC boards.  I prefer hardwiring my decoders unless they came factory equipped with a board.  Also, what address did you set it to?  It's possible that you set a long address but CV 29 is still set for a short address.  It will STILL accept programming commands but will not run on it's long address.  If all else fails, hook up lights to the board.  Digitrax decoders FLASH the lights alternately when a short or something is wrong.

Try This

If resetting the decoder as Nelson described doesn't work, try removing the decoder and testing the mechanism on straight DC.  It's possible you have a broken wire or other failed connection at the motor.  If you have a factory shorting plug for the 9-pin harness, add that and try the loco on DC.  If it works, you may still have some kind of decoder issue.  Otherwise, see if the motor runs with leads connected to the motor terminals.  If it does, you probably have a broken wire between the plug and motor or bad solder joint somewhere.  Sometimes there are bad joints where the harness attaches to the PC board, and re-soldering the black and/or red wire for the harness will take care of things.

Another problem with Athearn RTR trucks is blackener or paint inside the axle bearing slots (that can also transfer to the bearings themselves).  Carefully scrape the bearing slots in the truck side plates down to bare metal, and use some alcohol to clean the axle bearings.  I have two of the RTR SW1500s and not long ago I had to remove them from service due to erratic operation.  Cleaning the bearings and slots seemed to cure the issue.

Thanks for the tips!  I tried

Thanks for the tips!  I tried all of them and finally got it to work.  Here's what I did.  First, I reset the decoder, then reprogrammed it, and still nothing.  I tried two other different decoders - another DH123 and a DH163.  Next, I tried running it on DC - and it worked!  Then I tried to replace the Athearn factory DCC board with a Digitrax DH165 board, and still nothing!  

Then I tried the short address versus long address, and that actually worked!  I reset the Digitrax DH165 decoder (this fits better in the locomotive anyway) and tried the #3 address, and it responded to that.  So then I set it to #12 instead of #125 and it actually runs!

It's a very smooth runner.  So glad I figured it out, but not sure why it only accepts a short address, especially after trying so many different decoders.  Thanks for all the help, Nelson and Rob!

decoder

In Digitrax land 125 is a short address and not a long address.

Here's a video (not the best

Here's a video (not the best quality) of the train in operation.  The second SW1500 is a "dummy" (I took out the drive) as these will always run as a matched set.  I'll add sound eventually and will probably put the speaker in the dummy unit.

Try programming

CV 1 to 125.  Brian is correct in the fact that address 125 is still a 2 digit address.  Having said that, there are certain addresses that just will not work and 125 may be one of those.  It seams all decoders suffer from this.  There is information out on the web that describes this phenomonon. 

Interesting... I will have to

Interesting... I will have to try a different number near 125 and see what happens - perhaps 128, the number of the dummy unit.  Alternately, I will try programming a different locomotive to 125 and see what happens.  What are the odds that I stumble across that?

Here's a better video of the duo in action:

Address 128

128 is a long address - add 32 to CV29. (127 is the last short address)

Dante

Good lesson on short v. long addresses!

Tonight I tested out programming different numbers on different locomotives, confirming the short address scenario.  On JMRI, I have the option of selecting a "2-digit address" or a "4-digit address" for my Digitrax decoders.  Well, I always assumed that 2-digits really meant that and I would always use the 4-digit option for a 3-digit address.  This time I programmed the switcher to 125 using the 2-digit option and it now works on that address.  I have a locomotive 123 that I was trying to program a few weeks ago with the same dilemma and gave up.  I selected the 2-digit option for that as well and it works.

So I learned a good lesson here and it will be helpful in the future knowing the split between a long and short address!

Good lesson learned

Yes, I also learned that lesson once the hard way. In the same manner.  I never did figure out why 128 was the magic number to go to 4 digit addressing.  At first I thought it had something to do with hexadecimal numbers and maybe it does, but the largest two digit hex number is FF and that equals 255 in decimal.  The decimal 127 equals 7F in hex, so who knows?

Maybe some Digitrax wizard can tell us.

Regards, Vic B.

why at 127?

The DCC standard in document S9-2 shows (lines 35-45) that the first 8-bit byte of a baseline packet is the address for the decoder. When sending normal commands to a locomotive/multi function decoder, the most significant bit (the left most bit) has to be 0, which leaves 7 bits for an actual (short) address. 7-bits correspond to values 0 through 127.