They have a big network of sensors all round the track / ride. If any of them play up, the ride halts and doesn’t let anything go. Can be a broken sensor, or a random electrical glitch or whatever. Most often they just reboot the ride and the glitch goes away, and they have to run the ride empty once to check it is okay. If the sensor still shows something doesn’t go away, someone needs to climb up and look at the sensor and check the bit of the ride it is monitoring, which delays things for longer
Worst I’ve seen was the press launch for Saw at Thorpe Park, where it broke down with a bunch of celebrities at the bottom of the lift hill, half way round the ride, leaving them stuck there for ages. Ouch. There were some extremely worried men in boilersuits there. Although they got acres of coverage from that one, so I guess PR people win whatever happens.
We did some work on the PR launch for the Nemesis Sub Terra (with my “thrill scientist” hat on), I’ve ridden it about 10 times. It is one of those rides that you don’t want to know about before you do it.
Joe