This is one reason why you are not likely to be catching a Piccadilly line train in the centre of London for quite a while. It is a section of 38-pair telephone cable taken from the Aldgate bomb site by one of our engineers.

If you look out of the windows of a tube train you will notice loads of various cables hung along the tunnel walls. Apart from our telephone cables there are similar signal cables, HT cables, tunnel telephone wires, train radio leaky feeder aerials, and who knows what else.

Even when the train wreckage is all removed and the police have finished all their searches and the civil engineers have declared the tunnels safe, and the track gangs have inspected and replaced the rails, there will not be any trains until all those cables have been replaced.

And the way things are on the tube, our engineers will probably have to wait until the track gangs, signalling engineers and electrical power people have been in first, since they are more important than us.

Even cables without such visible and obvious damage will have to be inspected and probably replaced, and not all our cables are such small ones. Some of them carry 104 pairs of wires.

Its not often appreciated, but London Underground is an extremely safe place. They have hundreds of rules governing access to tracks and they have what is called a Railway Safety Case for each line. Unless they are complying with everything in that safety case, they can’t run the trains.

A lot of what is in the safety case depends on what is going through those cables and can include audio for passenger information points, long-line CCTV pictures from stations to the line controllers, audio for long-line PA announcements, information for platform information matrices, signalling control information, land line signals to train radio base stations, as well as all the bog-standard phone, fax and data uses.

That piece of cable in the photo is from Aldgate which is relatively accessible. The damage site on the Picc line is in a deep tube and will take even longer to get fixed – we haven’t even been let down there yet – and in such an enclosed space the damage is likely to be greater too. Not only that, but on the deep tube lines the safety requirements are even more stringent than for the sub-surface ‘cut & cover’ tunnels.

Basically, if we knew exactly how much damage there was, and had total access this moment, it could take many days of working round the clock to get everything re-instated – but we could be waiting weeks before we even get a chance to start work.

The one thing which will speed up the work is the absence of trains. Normally we can only work trackside in the four or five hours on ‘engineering hours’ each night when traffic is stopped. With setting up beforehand and packing up afterwards it would normally take a week to do 24 hours of work. In this case we should be able to do 24 hours of work in 24 hours.

I’m sure other parts of the Underground’s business will have similar tales to tell, and we are all gagging to get in there and get cracking on it.