This is more about risk management. They can run it empty or with fewer people but if a ride has a history of vallying due to a lack of weight, they are not going to take a risk of sending the train out without a certain number of rides and then having to evac potentially creating a safety hazard.
It's far less of a risk to run a coaster without people than to have your paying customers stranded somewhere in an uncomfortable situation.
It really is a bitch to evac a ride if it valleys, especially if it ends up coming to a stop where it's not easy to evac riders.
So asking for a specific number of riders kind of kills to birds with one stone.
1. You have a better guarantee that the train will in fact come back to the station.
2. You avoid the risk of having to evac a train in an not-so-convenient spot (may even have to require cherry pickers or even worse the local fire department) and avoid having to dis-assemble the train, winch it back, or remove a piece of track in order to take the train off. (One of those three usually has to happen.)
They normally figure out how much weight the train needs to make it around, let's say 2,000 pounds, and do an average weight per rider, let's say 175 and that would be 12 riders needed.
Sometimes this number can change due to weather conditions. Perhaps on a day where the winds are above 20 MPH they may need 16 or 18 riders.
(I'm just making these stats up as examples, I know nothing about the operations of El Toro, but I do know in general the operations of a coaster that needs x amount of weight to run.)
Hope this helps explain things.
--Robb
Very true Robb, never been to SFGADV but I went to Vegas in January this year, pretty much entirely to hit Desperado, and that thing can be shut down at the drop of a hat, for any reason. It took a lot of careful planning, but it worked out great even got a bunch of solo rides and the ride ops were telling us that the last time it was open which was two days earlier it required 17 people, before it would leave the station due to wind. I'm not sure how many of you have been out to State Line to ride it, but there are certain days were they wont have 17 patrons buy tickets to ride desperado, in an entire day. You have to really plan, sometimes you also just have to get a little lucky too, especially when it comes to wind.