By Tom Symonds
BBC Transport correspondent
If the 21/7 bombers had succeeded, it is difficult to imagine the effect on the city's transport system and its travellers.
Fireman Angus Campbell confronts Mohammed after his detonation
Two weeks before, bombs had gone off on three Tube trains and a bus.
The loss of life, and the sheer horror of the attack, in such a confined space, shook many commuters.
In the days that followed this became a deep sense of unease about the security of the system. On trains, passengers with oversized rucksacks were met with worried glances.
Passenger numbers had dropped, with up to a third deserting the system.
The immediate response to the threat by the British Transport Police was to increase the number of officers patrolling stations and trains.
Armed police officers used stop and search powers heavily - picking out young Asian men from the passing crowds for special attention.
Sniffer dogs appeared, and plain-clothed officers mingled with passengers.
But the truth is that the London Underground is one of the the most difficult terrorist targets to protect. Security is all about creating sterile areas where people can roam free, the risk they pose having been assessed, checks having been carried out.
To create such an area, it is vital to control the access points. Yet the Tube has hundreds of ways in and out. It is anything but a closed system.
Three million trips are made every day, and the system only works if the flow of people and trains keeps moving.
A dip in Tube passenger numbers has since been reversed
Putting security staff at station entrances to check bags would have brought the Tube and therefore the city to a standstill.
A Transport for London spokesman said: "7/7 demonstrated how difficult it is to prevent a determined suicide bomber."
And the airline security alert in 2006, following the discovery of an alleged bomb plot, demonstrates perfectly what would have happened if the Tube had been sealed up.
When it happened at airports, there were endless queues at security, and unbearable delays for passengers. Even now the effects are being felt.
In October 2005, perhaps with this problem in mind, the government announced it would trial new security technology on Britain's transport systems.
A full-body scanner duly appeared at London Paddington Station, capable of spotting objects hidden beneath clothes using "millimetre-wave technology".
To prevent holdups, only some passengers were invited through the scanner. The trials were designed to see whether the system could cope with working for up to 18 hours a day among Paddington's diesel fumes.
The bombers were able to escape along with other passengers
Other tests have been carried out - of portable scanners, and barriers to stop potential suicide bombers in cars at Victoria and Waterloo stations.
Earlier this year, the government revealed it was planning to release a substance called sulphur hexafluoride into the air within St John's Wood Tube station.
The non-toxic odourless gas was to be used to see how air moves within the station.
Apart from this no details were made public about what the government was up to - but it is understood this trial was examining the possibility of installing devices that can sniff the air for toxic releases.
'Range of ideas'
It is possible. The metro system in Washington DC has detectors in some stations, and its workers have been trained in the use of handheld sensors and escape masks.
London Underground's unions have in the past criticised the Tube for not adopting similar security measures.
But don't expect Transec - the security division of the Department for Transport - to publish the results of these trials.
They are just the tip of an iceberg, and below the surface experts are working on a range of transport security ideas.
In the end, the job of preventing terror attacks on the Underground lies in the hands of the security services.
Tracking the 2,000 or so suspects said to be at large is the first line of defence, and stopping them planning future days like 7/7 and 21/7 is the second.
Yet London is a resilient city. It didn't take long before the passengers, scared off by the attacks of 2005, came back and the tension eased.