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.
