I just looked at my heavyweight coach, item 13708.  I have two of these coaches and two of the combines.  Bought all of them maybe around five or six years ago and they were new .  I was surprised to see I looked today that the trucks appear to still be die cast zinc alloy. I did not expect that.

Being able to disassemble stuff is an integral part of model railroading.

Big 4, don't purchase a "DCC ready" locomotive unless you want to install the DCC decoder yourself. That term only means that the engine is ready to receive a Decoder.  And that itself is a maybe, because there may be incidentals required, depending on the decoder, I don't know them all. I am, however,certain from reading your posts that you are a beginner who would find decoder and sound installation daunting, frustrating, and quite possibly something to run you out of the hobby.  I've seen that happen to beginners trying to solder track when they should've bought sectional track.  Or taking on other tasks that are too advanced for the very beginner.

Make it easy on yourself right now.  Purchase a DCC system and a locomotive with a decoder and sound already installed right now.  Leave the DC behind.  I know that DC represents an investment, but at this point it appears to be an anchor dragging you down.  All is not lost with your DC stuff when you go DCC.  Keep the power pack for later on to provide power for accessories you don't have yet.  Keep the DC locomotive because you can get a professional to install a DCC decoder later on.  I promise you, when you hook that DCC system up to your track and put that DCC/Sound equipped locomotive on the track and you start hearing those sounds before the engine even moves, you will never ever go back to DC, which will be like pushing a corpse around the track.

DCC can appear daunting but the system I bought had great instructions for set up and the DCC/sound equipped Bachmann steam engine I bought was literally ready to go the second I put it on the track.

You should also know, FYI, there  are now decoders that allow you to run your DCC train using an app on your smart phone.  I just acquired five or six of these decoders.  The beauty of these is that you can take your engine and run on anyone else's DCC layout, regardless of whichever DCC system they may have, you don't need to bring a throttle with you or have to use their system that you might be unfamiliar with.  But I am ahead of myself.  Keep all of this  in mind for a future adventure along your model railroad journey.

There are several other modelers on this forum with long experience in model railroading.  I've been at it since 1961. If you have any questions, just come here, ask, and we'll help you out.

At one time Walthers offered a variety of trucks appropriate to these cars.

I have four of the newer heavyweight cars, if you call new being acquired new in the last six years or so, And I've had no problem with the trucks.  Of course I haven't run them much lately so I need to look and see, but the trucks seemed fine last time they were out.  They are beautiful cars and if for any reason the Bach Man's trucks deteriorate, I'll have no qualms about replacing them with Walthers.

I just realized, the truck frames may be plastic.  I am out of town right now but when I get back home next week, I'll pull those cars out and check for you.

Operation of DCC Locomotives on DC powered track can be done, but I've said it many times before here, it's like riding a bicycle backwards coast to coast.  It can be done, but it's very awkward and most of the DCC features can't be used.  You won't get the full enjoyment either until you have both at the same time.

Soundtraxx's website doesn't have a CV list for the 0-6-0 but here is the one for the 2-6-0 which is bound to be very similar.

https://soundtraxx.com/content/Reference/Factory-Installed/Bachmann/Spectrum/bachmann_On30_2-6-0_spc.pdf

CV 49 and 50 appear to be the relevant ones.  Try inserting the default values shown in the list and see if that doesn't fix the problem.  If not, try flip flopping the values.

I'm pretty sure that the locomotive address is retained in the decoder in the locomotive.

I believe so but you have to make sure that you have the polarity right on both of those track connection or you will have a short circuit.  In other words, you have to get the plug into the terminal Railer flipped in the right direction. If you plug it in and the train doesn't run, flip 180° and replug.

First thing I'd try is soak all the sections in water with detergent for a couple of days, toothbrush it thoroughly, and then rinse with clean water.

If that doesn't work, Spray with isopropyl alcohol from the drugstore and let it set a while, Toothbrush it off, and then rinse with water.  I would test this method on one piece of track first to make sure it doesn't affect the plastic adversely.

One thing everyone forgets is the rail joiners. I would plan on replacing them, you'll never get them clean up inside and you'll never have good contact.  And don't forget to really clean the Underside of the rail where the joiners go.

Agree with Jeffrey.  I had a hard time, figuring out what the OP was trying to say.

Test with another track connection cable.  And make sure the the plug prongs are clean.  The cable itself is an often overlooked trouble location.

The correct railroader's term for this "delta" track arrangement is "wye."

If you damage the truck pin extracting it there's no loss at all.  You just have to fill the hole with styrene rod. Scrap plastic sprue from kits is perfect for this job. After the cement sets, re-drill with a pin vise to take a 2–56 screw.  If you use the right size drill, you don't even need to tap the hole, the plastic is soft enough to take the screw .  It's a much better mounting job than the cheesy plastic pin Bachmann used to use in the Jurassic.

If your time and money is at all valuable to you, you should send that engine to a professional to straighten out.  Unless you really like backfeeding each individual circuit at a time to see what works what, burning the LED headlights out because you got the polarity wrong, and re-soldering half of it because you didn't get it right in the first place.

If you really want to identify the circuits, I've heard people say that you test each pair of wires with a 9 V battery.  I've never done that though .  I despise electrical work and soldering of any kind, and I would much rather be running my Railroad than running down blind alleys trying to figure things out.  You'll probably spend about $100 if you send it to a professional, but you'll have two or three evenings back, no heartbreaking errors, plus something that's done right the first time.

If the train is for kids, the conversion car is best.  They won't be doing any switching or any consist changing or anything. And if one of them actually gets interested in real model railroading in the future, they can convert the older couplers themselves then.