Transport Correspondent Campbell Docherty assesses whether yesterday's terrorist attacks on the capital were preventable...
The wider mayhem unleashed by the London bombings, leaving one of the world's major cities paralysed, confused and vulnerable, was not just a by-product of the terrorist acts - it was their raison d'etre.
That is why they - whoever "they" turn out to be - focused on the London Underground.
It is an obvious target, a mass transit system moving three million people a day, with 500 trains moving at rush hour, in tunnels often hundreds of feet under ground.
The security precautions needed to defend a network of that scale from the actions of a handful of extremists would immediately mean it would cease to be an effective mass transit system at all.
And you don't have to look at London alone. Look at the disruption caused in the West Midlands.
Coventry bus station and ring road was closed for three hours, and high visibility patrols were put on Birmingham buses.
It is revealing that the Department for Transport's Transport Security Team website focuses on aviation. In a post-September 11 environment, it is an obvious thing to do.
But the lack of specifics about public transport tells a story that became a horrific reality in London.
It states: "Security measures have to take account of passenger convenience and expectations.
"We cannot impose absolute security if people want to continue to travel with reasonable comfort and convenience."
Dr Pat Hanlon, a transport expert at the University of Birmingham, was personally involved in yesterday's events.
His daughter had just left Tavistock Square, where the bus bomb exploded, and was walking to Euston Station when she heard the explosion at Kings Cross.
Her attempts to get home to Birmingham were then hampered by the knock-on disruption to the capital's railway stations.
"Unless you introduce such draconian security measures on the Underground that you make it unpleasant and unattractive to use, there would seem to be very little to stop this kind of thing," he said.
"It's a black day for transport. What really scares me is the fact they have succeeded.
"Does this open the floodgates for an extremists to target transport systems in other places, in Birmingham?"
The distinction between aviation security and public transport is marked. While September 11 focused minds and, so far, has succeeded in preventing a repeat, the atrocities in Madrid last year have been repeated in London.
Dr Hanlon said: "On airlines, you can control the people coming onto the plane and even then, as we have seen, you can experience difficulties.
" But public transport systems in urban areas are wide open. It is horrifically difficult to control and, to be honest, it is surprising it hasn't happened before in London."
Dr Hanlon believes the London bombs could make an underground system for Birmingham less likely to happen.
"Safety and security were not, until these events, at the forefront of thinking when it comes to building new underground systems, but it will influence people's thinking now.
"It is an element which counts against putting mass transit underground, although whether or not it will be persuasive when it comes to decision-making remains to be seen.
"It is certainly true that costs spiral when things like this happen.
"There is a natural feeling that one cannot economise on people's safety - the cost of human life is infinite - and this will now have a further effect on the scheme being championed by Birmingham."
As Dr Hanlon points out, transport is now in the front line in the terror war.
The nightmare scenario has happened and, it is abundantly clear, that nightmare was never far-fetched or fanciful.