I do not see anything obviously wrong with the wiring. My initial guess would be there is a large voltage drop on the rails when the dual motor diesel is running that is not present when the diesel is in neutral. Causes of voltage drop can be one or more of the following items. Voltage sag at the transformer output due to the 4A current draw of the diesel. Solution, possibly a higher wattage transformer, 100W minimum is needed. Too small of a wire gauge, 16 ga minimum is needed with a power connection every 6' of track. Poor connectivity of the 690 track clips. Clean the sides of the rail flanges with sandpaper, bend the metal on the clip for a tight fit and attach it using a conductive, antioxidant grease. Track pins may not be clean and tight. They should be cleaned, I use sandpaper, make sure they are tight in the receiving rail and again use a small amount of conductive grease on the pins.
The total voltage drop due to the above items can be around 3.5V at 4A.
