I don't have these specific models but it appears (from Lionel's parts pages) that they have recent generation Legacy smoke units with 8 ohm resistors and plastic reservoir bowls. The later (6-85215) units also appear to be equipped with a thermistor on the PCB board - not a component shown on the earlier board.
What you are describing reminds me of what happens when an AC regulator fails on earlier generation smoke units, as in full track power to the resistor and it and the PCB are smoked. But I don't think that can be the cause in your case as the components are different.
None of the Legacy engines I have with boards similar to yours have displayed the kind of failure you describe. They've been reliable in operation although the smoke output is not quite what I hoped for. Your experience makes me wonder if there has been some kind of maintenance or power supply issue. Have you kept the smoke units fully topped with fluid? What transformer are you using?
