Or did you leave a metal tool laying over the track? 

Or have you tested it without the locomotive, to see if the locomotive is the problem.

Or, as can sometimes happen, at either end of the power feed to the track, do you have a bare stranded wire where one loose strand from one wire is touching the other wire?

Or, is some freight car causing the problem?

I don't know about other brands of decoders, but Soundtraxx decoders have CV 113, which has two functions. The first is that it keeps the sound turned off until you either move the engine or activate a function like the whistle or bell. The second is that you can set a timer at any interval you wish to turn the sound off once it's on and the engine standing still. So I never have that problem with multiple inactive engines creating unneeded sound. Hot steam engines at rest make a whole lot of noise too, between the blower, the air pump, the injector, and the dynamo if the headlight is on.  And mine are all set to go silent after 5 seconds.

If you want only a couple of engines sound equipped, well, yes, a limited function cheap decoder will do. And I can see that in a diesel consist, really, only the lead unit needs sound.  I couldn't live like that though.  Every engine actively used on my layout has sound. And there's a lot of them. It took money and time to get there but I have what I want.

And, about the work bench.  At one house I have a card table, which is mostly adequate but collects too much junk most of the time and requires frequent clean off and organization. The other place, all I have is a TV dinner table. I have to say, it is too little for anything meaningful to happen on other than coupler and wheel set installations.  I have built a couple of boxcar kits on it, and the experience is totally frustrating, not having enough room for the tools needed, having to go into a toolbox all the time, and having stuff roll off.

I knew a guy in Baton Rouge with one of those giant triple-garage filling layouts.  He had a shop that was the size of a small bedroom. Multiple test tracks down one entire side, another side was a 12 foot long work surface, another side had a drill press, saws, and a paint booth. I don't remember what else, but it was beyond the wildest dreams of anything I might ever have.  The guy had retired from a Railroad management position so, of course, everything was super organized, with all the tools hanging in the right place on pegboard walls.

That's a great idea

Yes, I've had those issues on my much smaller layout, but with unlevel conditions due to house foundations settlement issues, endemic in our marshy region. But so far, shimming the only occasional errant coupler pocket has solved the problems on my Railroad. 

Coupler pocket dimensions vary between manufacturers. Some provide looser coupler fit than others.  Using Kadee's own pocket supplied with most packages of their couplers, would take care of vertical Coupler play but naturally, that is not really practical on most models.

If the couplers are riding up and down, they are installed too loosely in the pockets and need shimming so that they don't ride up and down.  I had that issue also on some cars, but a little narrow shim right across the front of the coupler pocket solved that problem for me. So the problem is not a flaw in the coupler.

Just FYI, couplers fail on the prototype too.

One night I was at a party in a fourth floor condominium overlooking the New Orleans public belt railroad. A long transfer move was passing below at slow speed. I wasn't paying any attention until I heard the air line pop and the whole train going to emergency, slamming together all the slack in every coupler in the train.  When I went out on the porch to see what was going on, I could that see the train had parted somewhere in the middle, with each section about 40 feet apart or so.  It was very obvious that a coupler knuckle had broken.  The train sat there for about 15 minutes until a couple of trucks came down the right of way with a few guys that got out and worked on the coupler for a while, obviously installing a new knuckle. After they got done, the front part of the train backed up to pick up the rear part, pumped up the air and left. No one at the party but me knew what had actually happened.

Agreed, Jeffrey. 100%.

And if "large scale switching" means coupling up of long cuts of cars to each other, then the weight of each cut of cars on either side of the coupling operation will guarantee a smooth couple-up with any coupler, even the horn hook.   I can personally attest that trying to couple only one single car to another single car car, both with horn hooks, will never, ever, ever smoothly couple up. The plastic springs manufactured on the side of those dogs are just too strong.  You'll push the car along forever and nothing will ever happen.

And, my opinion only, in my experience, Kadee knuckle couplers have satisfied every need on every operations layout I have ever had.  The appearance is much better than horn hooks, and with careful installation any kind of operation is flawless.  If they are pulling out or letting go under heavy use such as long cuts of cars, the problem is in the installation, not the coupler.

What is your operating system?

I've always paid a tech to do DCC/sound installation work. For 100 bucks or whatever the charge is, you know it will work right and you won't be frying any expensive components because you did something stupid somewhere. Or poking the soldering iron through the tender shell. Which I have seen done.

Also, I always thought that when you said "DCC," you also meant "sound."  It's beyond me why anybody would want to put DCC into a locomotive but leave it a silent runner. You can always turn the sound off if you don't like it, but somewhere along one someone will want it.

While I'm talking about sound, the other thing I don't understand is why some folks want to turn it down so low that you can't hear it more than 3 feet away from the engine.  I get it that some places have multiple engines running, and I'm usually operating by myself, but that soft volume thing is something I just don't get it. Real trains are LOUD, especially if you have ridden inside a steam locomotive cab at speed.

Call Soundtraxx.

https://soundtraxx.com/

They offer umpteen thousand DCC related items.

Plus, their customer service is unsurpassed. You will get a very knowledgeable person on the phone. They will know the solution to your problem.

The Bachmann catalog has many DCC/sound, equipped, locomotives available, plus a variety of DCC operating systems.  I would not at this point attempt to purchase separate DCC components to install into a locomotive yourself. That is a task suited for more experienced modelers.

Cars that are too-free rolling can be a problem on layouts that go beyond continuous running and become involved with switching. If the siding where setouts occur is not dead level, the car will unwantedly roll. Great care in construction is needed to prevent this. However, I live in an area with terrible soil subsidence. The house is always settling randomly and my once-level lay out now has dips here and there.

This can be a problem on the prototype as well. Devices called "derails" are installed at such locations that flip up over the top of the rail to stop errant cars from rolling out onto the main and fouling it.  Model derails are available, and I have several that I unfortunately have never gotten around to installing in the problem locations.

Long ago I heard a tip to slow these too-freerolling model cars down. To do that, you cut a Kadee coupler knuckle spring in half and put it in the journal of one side of the wheelset. For someone like me that doesn't run very long trains, it's a good idea. However, it is one that I have not gotten around to yet.

I might guess that larger wheels rotate fewer revolutions per mile, eventually presenting some savings on bearing wear.  Freight trains are much faster today than they were in the days of 33 inch diameter truck wheels and friction bearings.

Master model railroader is a title bestowed upon NMRA members who have completed an exhaustive set of set achievements. This achievement program did not exist while John Allen and many of the pioneers were alive.  Today there are 700 some odd people who have earned the title.

Nevertheless, Terry, there are undoubtably many many excellent model railroaders who could be called master model Railroaders who are not NMRA members and who do not engage in this program, but have actually accomplish many of the program goals set forth.

Nice railfanning plan.  Thanks.