What board was in the damaged engine? PS-2 5V or 3V or PS-3? There are board faults that occur that cause the lights and couplers to get full DC voltage from the board. That would be the only thing I can think of that causes both couplers to melt along with lights.
Remember TVS is just clamping the AC input to the device protected based on what is being discussed here. The PS-2 boards actually have TVS on them, the TIU has them on AC side and also on the Var track FET circuit. Transformers have them on output.
It clamps voltage spike. Not a high current inrush from a derailment where chassis ground contacts center rail as an example.
It also won't clamp any DC output side faults, such as a pinched wire, or when a bulb blow naturally. That is why every so often a bulb that goes bad can take out a fet, or in worst case scenario short a fet and momentarily cause high rush current that can burn a trace.
Every so often I get boards that require a jumped trace after fault. It happens enough I know where to look for the pinhead burn hole on the PCB. G
