MTH Engine truck grounding problem

You either have a Coupler PV wire frayed and causing this, or a broken AC power wire from the truck causing this issue.  Look at wires much closer.  Even a split or fine crack in the insulation can cause this.

Unplug the coupler wire can help isolate this.

Also make sure no frayed wires on the motor lead.  Also a common problem for older wire.  If white is frayed engine moves immediately even while in neutral.  If yellow frayed in forward you can get sparking.  G

Thanks for those pointers.  I will check again.  Can you confirm whether the main chassis is supposed to be electrically at the same potential as the trucks?  I am able to tell that the motor power supply is a bridge rectifier.  The AC input side of the bridge is connected to center and ground rails.  The output side of the rectifier has a B+ and a B-.  By my theory , that B-, or "common" cannot also be shared with the "ground" on the AC side of the rectifier.  They would have to be insulated from each other.  If they are shorting together , as in my case, then 1/4 of the bridge rectifier is being shorted out.  So I am trying to figure out where the insulation is that isolates AC ground from B- common, and it has something to do with the chassis being grounded or not.

The chassis and trucks are AC Ground outer track for 3 rail.  The rectifier makes DC.  Yes that + DC is motor drive and also lights and couplers.  But the return is through FETs, also there are several other DC circuits on the board.

PS-2 board should never have DC tested against AC, you risk blowing the board if you touch the wrong point.  Because the PV (+DC) is made from the rectifier that is why you can see sparking from chassis if any of those PV lines (normally purple) to the smoke element, lights, coupler, or motor touch chassis.

The 8A rectifier can take that periodic short.  G

GGG, this is a significant help to my troubleshooting. There is obviously a DC ground plane on the board. I am seeing that ground plane as electrically connected somehow to the chassis (AC common) through the board’s mounting hardware, which shoukd not be. DC ground has to be isolated from AC common. With all power OFF, my ohmmeter confirms that those two circuits are NOT isolated and shows zero ohms. When I discover where and why this is, I will be 90% to solving this. Perhaps there is a bypass capacitor somewhere on the board that is shorted.

SOLVED!  Thanks , GGG, for explaining the grounding.  What I found was a short between the BLUE wire of one of the two adjustment pots that sit on either side at deck level of the chassis on their own small PC boards.  I don't know exactly what these pots do.  The troublesome one is NOT the one that says "smoke". The one causing the problem was on the RIGHT side facing forward and its BLUE wire was shorting against the metal bracket that forms the mount of the bridge rectifier above it.  It looks like the factory attempted to prevent this by putting black insulating material on the bracket.  However the small pC board containing the pot had a solder blob on it with a very sharp point which punctured the factory insulation, creating the short. The way I discovered this was to measure for continuity between pin 4 of the bridge rectifier (DC -) and the chassis, and then progressively pull connectors. Once I found that pulling the longest connector (12 pin) made the short go away, it was just a matter then of finding which of the 12 pins had chassis ground continuity to it, and it was the blue wires. I deburred the sharp solder point with a file and put some electrical tape over it so now there are two layers of insulation keeping it from touching the chassis frame bracket.  I have attached a photo showing where the short was.  The black in the background was the insulating tape the factory had put on the bracket.  The black tape over the PC board is what I added after deburring the solder blob.

Now I need to cut my losses by finding a buyer for the two motors I bought prematurely when I originally thought the motors themselves were sparking. For anyone interested, I have a BE0000152 and a BE0000153.  The location of the sparks was where the chassis came in contact with the front and rear trucks, because there is pivot movement between those two mechanical parts, causing the sparks (and leading me on wild goose chase thinking motor issues).