I'm going to give you the useless answer - we don't know for sure. The number of factors is very high. Without running any sort of model (or a decent source), any figures anyone
actually gives you are likely a total fiction.
- Wind speed (as you ask).
- The direction and aerodynamic effect of the trains - The crux of this is that the specific shape of the train and direction of the airflow (no just compass direction, but the vertical speed, turbulence, etc) have a huge effect. The drag forces of the wind on the train are staggeringly hard to calculate. At best, you'd have to look to the manufacturers for some of this data.
- The temperature - Not just the air temperature, but also the wheel/bearing temperatures and things like that.
- Is the wind constant (unlikely), or gusting (if so, how much)?
As a quick estimate, it would be fair to say wind speeds approaching the speed of the train over the crest of the hill would start to raise alarm bells, but those bells are probably already tingling at much lower wind speeds if conditions are right.
The manufacturers probably run some calculations during the designs stage that say something like:
Assuming a loaded train, and standard operating conditions (temperature, etc), the ride should not operate if the wind speed is gusting over 35mph in a SW direction.
More sophisticated rides like Dragster (not that PMBO isn't sophisticated, but... come on...) actually record the train speed over the top of the hill and adjust the launch speeds of subsequent trains accordingly. Obviously can't quite do that on a lift hill, but the point is that the designers have ways of understanding this.
