[TheOtherHobbes]

One of the actual reasons is that drivers go through intensive training (er...) as first-call engineers.

If a fault stops a train, it's far more useful to have someone on the scene to try emergency repairs than to wait until the train can be cleared from a tunnel - because a stopped train can literally hold up the commute of millions of people.

There are also safety issues. Drivers are trained in passenger management, and every so often they need that training to deal with fights, illness, suicides, or all the other messy things that happen on a public transport system - the public part of that being at least as important as the transport part.

[michaelt]

  Drivers are trained in passenger management, and every so 
  often they need that training to deal with fights, 
  illness, suicides, or all the other messy things that 
  happen on a public transport system

As I understand it in a lot of the London trains it's not possible to move between carriages - so the driver can only deal with fights, illness, suicides etc over CCTV or by stopping at a station?

[yangyang]

There are doors but they are not for use in normal circumstances. The new Victoria Line trains were initially supposed to be "space trains", open all the way through, but didn't actually end up getting built like that.