george matthews wrote: to inform the driver when it is safe to depart.
Indeed, and that language is carefully chosen. The fact that it is "safe to depart" does not in itself give the driver authority to proceed. She is still governed by whatever system of train control is in use, and thus needs authority from the signallers. This would normally mean that she needs to see a fixed signal at 'proceed'; if the block signal at the end of the platform is at red, then the guard can sound the buzzer as long as he likes, and the platform staff wave all the white bats in the world, but the train cannot proceed.
george matthews wrote:The London docks area has an automatic metro... Control of the train is managed by a computer which oversees the whole line.
Yes, the Docklands Light Railway, which has proved remarkably successful. It uses a moving block system of train control. I saw an interesting (albeit populist) little video about moving block on the DLR just a few days ago -
https://www.youtube.com/watch?v=a52ZSCIAoRk" onclick="window.open(this.href);return false;