look here.  Joe
https://en.wikipedia.org/wiki/Distributed_power

Will this help?  Scroll down for photos. Joe
http://www.mexlist.com/40foot/PartV.htm

I would e mail Soundtraxx tech department. Joe

The lens should come out. Put the shell in the freezer over night. This may loosen the glue on the headlight lens. Use a small set of needle nose pliers and gently wiggle the lens assembly back and forth on the inside of the shell. I removed the lenses this way on several GP9 shells.  Joe

The board with an 8 pin plug are interchangeable. You then buy a sound decoder with an 8 pin wire harness on it. I prefer using a board style like Soundtraxx uses. It's a better install in my opinion.     Joe

Looking at the diagram, there might be room for a speaker in the fuel tank or in the indentation in the frame top towards the rear. Soundytaxx has a board style decoder but it is not a plug and play.   Joe
diagram: http://www.bachmanntrains.com/home-usa/dwg/dwgs/H609X_IS001.PDF
Soundytaxx  decoder: 828041
Look at this link for an idea of how to put in the decoder board
http://www.soundtraxx.com/documents/appnotes/bachmann_gp7.pdf

Remove the screw holding the fuel tank on, remove tank cover. There are 2 screws diagonal from each other in a recess. Remove these. Shell will come off. It's a tight fit.  Joe

The voltage for HO should be 12 to 16V DC max.  Sounds like the power pack is bad.   Joe.

NCE and Digitrax both have entry level systems that are expandable. Or to get going now if you have a computer, Digitrax sell a PR3 programmer. It hooks up to a computer. You will need JMRI decoder pro software to use it on non Digitrax decoders. JMRI is free.   Joe

http://www.modeltrainstuff.com/Digitrax-PR3X-Decoder-Programmer-p/dig-pr3xtra.htm
http://jmri.sourceforge.net/download/index.shtml

If you're talking about one for a reverse loop, then yes it will work with any DCC system.   Joe

It could be a mechanical problem. If you want to reset the decoder, is there a hobby shop close by with a DCC system they can use to reset the decoder? If not state the city and state you live in to see if someone close by can help you. You can take the shell off and see if you can turn the motor by hand in both directions. If one way works and the other way doesn't, it's a mechanical problem in one or both trucks , which means dis-assembling the loco  to see if you can fix it, send it to Bachmann to fix or is it under warranty? If under warranty, send it to Bachmann  get fixed for free.  Joe

Is this the one you bought?
http://shop.bachmanntrains.com/index.php?main_page=product_info&cPath=264_274&products_id=1819
If so you need this
http://shop.bachmanntrains.com/index.php?main_page=product_info&cPath=264_274&products_id=1821
and this
http://shop.bachmanntrains.com/index.php?main_page=product_info&cPath=264_274&products_id=1822
Good luck finding the companion. Bachmann is out of them and those on E BAY can be over priced.    Joe

Example of undecorated   Joe
http://www.modeltrainstuff.com/Atlas-Master-HO-Alco-S2-Undec-DCC-p/atl-10001482.htm

CV8=8. A value of 8.Once you program it turn the  power off to the track wait 20 seconds or so  and turn the power back on, or lift the loco off the track wait 20 seconds and then put the loco back on. If the re-set took, the headlights will flash. I can't remember how many times, maybe around 20?  Depending on you DCC system, the default address 3 could be 3,03,or 003.  Joe

Since you're using 18" radius track, go with the 0-6-0 or 2-6-0 DC model.    Joe