Yes, it does happen once in a blue moon. But when it does happen, you need a trained member of staff.
Faults happen fairly often. If you've got a crush load, peak service with a fault, with no member of staff on board, what're you going to do as a passenger? The priority is to get the train moving so you don't have people stuck in tunnels - when that happens, people tend to faint. A passenger can't be responsible for themselves in that situation because they won't be able to do anything, they'll be standing on a broken train in a tunnel. You could argue you could have a fitter at every station who would walk to the train. Then you've got the time it takes for them to be alerted, them walking to the train, looking at the systems in the cab to work out whats wrong, fighting their way to the correct part of the train to correct the fault - it would delay it all by too much.
A situation to look at is, summer time, rush hour, crush loaded trains, 3 trains in a tunnel between stations. The lead train has something simple go wrong - a blown fuse - but the safety circuit has been tripped. You'd need to get a member of staff to the train to reset it all, and replace the fuse - this can be done easily, fair enough. However, in the time it's taken for this to happen, two people have fainted on the 2nd train - (before you say that's once in a blue moon, it's a daily occurrence at many stations). Someone has pulled a passenger emergency alarm. The train has lost it's round train circuit, needs to be manually over-ridden. The third train has been sitting in the previous station, causing trains behind that to queue up in tunnels. All of this has delayed the service, creating bigger gaps, longer dwell times, less space on trains. You need quick action to get a train moving to keep the service going to transport passengers.
I'm not trying to play a trump card. As I said previously, before I commented, "Having just completed a 3 week train operators course...". Before that, I was under the impression that train operators were lazy, incompetent, didn't see the big picture whatever. Only after having sat a trains course myself do I see what they have to know to be sitting there, and see why they are needed.
