With the introduction of magnetic braking, one would imagine that operation in a downpour would be safer, as friction is no longer required. However, during OSU Day at Cedar Point this year, Cedar Point kept GateKeeper at 1 train operation through rain, just to minimize the chance of the final friction brakes/tires not stopping the train.
My guess is that this is more a corperate/legal "we are doing something to ensure the safety of our guests" thing than a genuine concern. So that, in the event of something happening, they can show from their records that they did everything in their power to prevent such a thing. And post incidents, they need to make it look like they've done something to prevent it happening again to those in the know, I guess?
Some parks introduce procedures directly post accidents that then fade away and get forgotten about, because the parks know full well it's stupid over-caution for a freak occurrence.
It's like that whole no more duelling on Universal's dragons thing. If that was a legitimate concern then their duelling coaster in Singapore would no longer duel. And it's the same with Cedar Fair. Apparently, only at Cedar Point is there a risk of the rare possibility of a train sliding through the breaks! At neighbouring Kings Island, it's impossible!
Speaking of which... I spent two days at CP when I went. It drizzled a bit. They closed several rides and put everything on one train. I asked an attendant at Maverick, because at the time I was genuinely oblivious, why it was down expecting them to just say "technical difficulties"... He said "cuz of the weather". It wasn't even raining, at all, at the time. I went to guest services to look for an explanation and they told me that the
manufacturer guidelines state that Maverick cannot run in the rain and it MIGHT rain... I didn't know whether to laugh or cry. The following day we went to Kings Island. It poured in the morning and then was drizzly, dark storm-cloud cover all day. Everything running as normal.