The Bachmann decoder is a basic non sound. The other is probably a SoundTraxx Tsunami 2 TSU-1100. The SoundTraxx web site will have instructions for installation. https://soundtraxx.com/products/tsunami2-digital-sound-decoders/tsu-1100/

I'm late on this, so you might already have your solution. I have two of the 2-4-4-2 locos. One has a TCS WOW 21 pin installed, note: the motherboard has to be moved rearward in the tender due to non standard pin arrangement. The TCS board has to be turned around to fit. The other 2-4-4-2 has the 21 pin 0-6-0 Bachmann/Soundtraxx 44959 module installed, the 44947 from the 2-6-0 is the same. It works fine, just plug it in. Next go to  https://soundtraxx.com/reference/factory-installed-decoders/bachmann/  The 2-4-4-2 will be listed, the CV values for it are there. Print out the table for that and for the 2-6-0, you can the compare and see what CV's are to be changed. The 2-4-4-2 already has a speaker installed. Hope this helps.

   No "shorting plug", this loco has no tender, so all the electronics and speaker are shoe horned into the loco itself which would be hard to get at. Also no way to put another decoder in except with lots of work.

   I didn't mess with the CV's since I only run DCC. But, I decided to do it. If you program CV 29 to a value of 38 it does enable the operation with DC also 28/128 speed step and loco address 128 or higher. If you were to use a value of 6 then 28/128 speed step and DC operation are enabled. The question of course is why isn't DC enabled in all of these trench locos?  Default on a full WOW decoder is to have DC off. If you don't have DCC, you can't program CV 29, and if you only have DC you wouldn't have a clue what's going on. BTW, the trench loco doesn't really like the MRC 1370, no slow speed control, jerks a lot. I had to use the instruction for a full TCS WOW decoder to get these values. So I guess this problem is solved.

   I'm using a MRC 1370. I take it that the B-mann power pack is filtered. (Oops, guess not)Well, I just tried it with the B-mann gray power pack and it doesn't work either.  The 0-4-2 with sound works and the Heisler works but not the trench loco. Over on the Facebook On30 Raillroading, someone got theirs to run with the standard gray powerpack, but, I can't. Don't know what's going on.  I just got out my old MRC Tech 4 Railpower 250, same results except the Heisler operates all wonky, hit or miss on the sound, random reverses with throttle control. I think this is a pulse modulated power pack which is probably the problem with the Heisler.

   Here's a question, how many have tried the trench loco on DC? Seems some of them sent to England won't work on DC. I tried mine here and it won't run on DC either, a Porter 0-4-2 runs just fine. Other people have said theirs runs on DC. I haven't messed with CV's since I will use it only on DCC. But it is odd since Bachmann locos usually are dual mode. Seems like it is random. Any ideas?

   I'll add this. I just got (3 weeks ago) a "new" old Shay and the gears are not the same as an earlier one that I have. The box was sealed. The bevel gears are definitely much beefier than the older ones. I have run it about 50+ hours so far with a number of times 6-7 hours straight. No trouble so far.  :)So there has been a change in them. So I'm not sure about the issue, have they been improved? Seem to be.

Ken Trossen