Northern Line Closed
The problem lies with the tripcock system. obviously no train driver deliberately passes a red signal, but just in case we do, there's a fail-safe emergency brake system. This system is basically a lever next to a signal called a Train Stop:
Anyway, this train stop will catch onto the tripcock hanging below the front of the train if a signal is red. When operated, this will cause the train emergency brakes to apply, thus safely stopping the train quickly. If the signal is green, the trainstop is lowered and the trains tripcock passes over the train stop without problem.
(Please note that my description of the system is extremely 'laymans' to avoid confusing non-technical types! If you'd like to discover more about the tripcock system, you can read the 'signalling' pages at District Dave's website, from where the above photo of a train stop came!)
On the Northern line, it has been discovered that some of the trains are not being 'tripped' by this system when a signal is passed. This is obviously extremely unsafe. As I said, no driver will deliberately pass a signal at danger, but if he does and the tripcock doesn't work, then there's a very high probablity that the train will meet the one in front and a collision would occur.
We obviously do not want this to happen, therefore I fully support my colleagues on the Northern line in their actions of refusing to work on the grounds of health and safety. I know it's very inconvenient for you commuters, but I hope that you'll understand why this is happening and will support the drivers.