Bullshit, I don't know where you heard that from, but fiberglass is an insulator when there are two different metals touching each other, to prevent corrosion.
There is no place on these cars where two different metals touch each other and need to be insulated between the layers. I can't think of a single legitimate reason to use fiberglass to repair metal on one of these cars (replacing a whole panel is a different story). These cars don't use rivets or bonded construction, and we're talking about using fiberglass to cover metal repair that's been done poorly/incompletely.
Other industries and applications are different, aircraft are a completely different ball game. Multiple layers, insulated panels, etc, are a different story. But none of that is relevant to this application.
