I would assume that they have, and that either they took care of it, or MMA never needed to do any real engine work on them (which would be my guess, B23-7s are good engines, and Santa Fe and BNSF had overhauled them).
In fairness, the C30-7s were also in excellent shape when MMA first acquired them and they had been very well maintained up to that point.
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Considering that the Bondo repairs didn't hold up for very long, I suspect CMQ would've had to address them by now if they were done to the B23-7s, keeping in mind that those repairs would've been made at least three or four years ago. And the consequences are quite well known, given the publicity and notoriety surrounding the disaster in Quebec, so it's not as if CMQ would've been unaware. I can't imagine that CMQ management would be so foolish as to not check for stuff like that. Presumably they don't want to be another MMA.