This does happen to PS-2 5V boards (8.4 Volt Battery). Why is not really known. If you power it up conventionally with you Z-4000 monitor Amperage. If amps rise with voltage and get over 2 amps towards 4-6amps with voltage up at 11-14Volts, there is a pair of diodes between the board that have shorted AC input. Effectively this kills the board as they can not be separated to repair without expensive equipment.
Why it happens has not really been analyzed, but these boards fail more regularly then their 3V counter part. I "believe" some of the components just did not have long life under the condition trains operate, that battery condition may contribute as the board has to draw greater initial current to charge the battery. Really hard to be specific though.
You may also find that the 330uf capacitor is swollen. This seems to occur when the diodes short, because I believe the polarized capacitor has the voltage potential reversed in the failure.
At some point with enough applied current the diodes will open, and at that point the board still is non responsive with no current draw when powered up.
One of your alternatives is to have it repaired with a PS-2 3V board with 5V connectors. This includes changing to a 4 ohm speaker and the 2.4V battery. G
