It sounds to me like your MRC Prodigy Express doesn't have sufficient power output for sound locomotives and its internal overload breaker is kicking in and out. You need to see if it can be upgraded to a higher amperage power supply.
It's also possible that you have used too small a gauge of wire between the Prodigy and your test track, and the wiring can't handle the load, which is subsequently causing the Prodigy to overheat and turn off.
Are you putting the engines on the track one at a time (i.e. powering up the decoder in one engine at a time) or all at once. I can't believe your system can't run a single sound engine unless your system has a problem. Does the Prodigy Express have separate track and programming track outputs? And is your test track wired to the track output, not the programming track? To run the engines, you should have the track connected to the track outputs.
Jim
I agree with Jim. I run the PA and the program track doesn't put out enough power to do anything. I believe the Express is 1.75 amps, which should run three newer locos with sound.
Tilden
In my experience, track that is super clean, like with alcohol, becomes dried out and the contact problem can return. I put just a very small amount of WD-40 on each rail and let the loco spread it around on the layout. In your case with the test track, just spread it on the rails with one finger and wipe the excess off with a different finger. Most light oils, like WD-40, improve electrical contact. If I start having trouble with a loco, I clean the wheels, lube the gears, and apply a small amount of WD-40 on the wiper contacts and run the wheels through a bit of WD-40 on the rails. I do have to make the distinction between dirty track and wheels versus dry electrical contacts, but most of the time the above procedure takes care of the problem.
Hope this helps,
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
Actually, totally dry rails cause more electrical sparking, which pits the rails/wheels and creates a lot of the black gunk you see on the rails over time.
I've found if you clean the rails with mineral spirits and then don't try to get the rails absolutely dry, but leave a microscopic film of the mineral spirits on the track -- the track stays cleaner longer. The ever-so-slight oily film from the mineral spirits actually reduces the sparking, so the rail gets pitted less and there's less black gunk formed since there's less sparking.
Another trick if your track is already clean is to moisten a cloth with some mineral spirits and rub down about 4 or 5 feet of track with the cloth. Leave a good amount of the mineral spirits on the track and then run trains around your layout, letting the wheels of your equipment spread the mineral spirits around the layout.
If you turn down the lights, you will see less sparking, which translates directly into less pitting of the rails and wheels -- and less black gunk being generated.
Joe Fugate Modeling the 1980s SP Siskiyou Line in southern Oregon