[simpleigh]

Care to share any?

Reasons I can think of:

Existing automation may be insufficiently good to cope with adverse weather and unplanned situations. Technology has moved on considerably since the Victoria, Central and Jubilee stock. We're automating cars now, after all. The article mentions a new signalling system being rolled out in support.

Lack of platform-edge doors. TfL have stated they won't run driverless trains without these. These are planned for installation on the Piccadilly line as part of the new stock rollout.

[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?

[sthulbourn]

The line is equipped with an Automatic Train Operation system (ATO); the train operator (driver) closes the train doors and presses a pair of "start" buttons, and if the way ahead is clear, the ATO drives the train at a safe speed to the next station and stops it there. This system has operated since the line opened in 1968, making the Victoria line the world's first full-scale automatic railway.[20]

http://en.wikipedia.org/wiki/Victoria_line

[ealexhudson]

Platform edge doors is definitely part of it - people get stuck in the gap, children fall through, there's few practical driverless systems that can cope with that. Yet there are some places - e.g. Central line - where the gap is almost a foot between train and platform. The doors aren't the only problem, it's the whole carriage, on those curved sections.