Hi-tech systems for detecting terrorists are being tested throughout the rail and Underground network, the Transport Secretary, Alistair Darling, has revealed.
Although the Government refused to confirm the locations, it is understood to involve the use of sophisticated CCTV systems.
And in the new year airport-style scanning equipment is to be introduced on the Heathrow Express rail link as part of a pilot scheme to detect suicide bombers.
Passengers on the high-speed rail link between the airport and Paddington station will be asked to co-operate with a range of screening systems such as the "millimetre wave" equipment already in use at Heathrow's Terminal Four. Volunteers will be asked to step into a telephone-box-size booth where the apparatus will be able to detect anything concealed under their clothes.
They will also be asked to pass their baggage through X-ray machines. A hand-held device for detecting explosives will also be used. Sniffer dogs will be on hand to detect any suspicious material.
Mr Darling is in talks with London Underground and the national infrastructure company Network Rail about pilot schemes using similar technology, although the body scanner is unlikely to be used.
Baggage reconciliation, which involves rail and Tube staff asking passengers to account for any suspicious items of luggage, is also being looked at.
Mr Darling told a conference on international transport security at the Queen Elizabeth II centre in Westminster yesterday that the new technology could be used on a "selective basis".
He said: "You can't replicate on the railways or the London Underground what you do at airports, where you have a sealed system.
"If you go to airports, everybody goes through the screening. Your luggage is screened, all the staff, everybody going airside gets screened.
"Heathrow airport is the busiest airport in the world but Waterloo station carries about four times as many passengers every single day. Three million people a day use the London Underground system. You simply couldn't have people queuing up for hours to get through. You would be doing the terrorists' job for them.
"What you can do is ask yourself whether, on a selective basis, at a point where it is appropriate, it could help to make things safer and reduce the risk."
A London Underground spokesman confirmed that a trial of an "intelligent" CCTV system had been carried out at Liverpool Street station in 2003.
The technology is programmed to look for suspicious patterns of behaviour, such as an individual standing in the same place for an unusually long period of time or a bag being left unattended.
The spokesman said the system tested at Liverpool Street had a tendency to produce false alarms. But the technology had moved on "significantly" and the New York subway was about to test an updated version, which London Underground was watching with interest.
Michael Brown, the Tube's chief operating officer, said the number of CCTV cameras on the network would be doubled from 6,000 to 12,000 in the next five years. By 2010 all the cameras will use digital technology and will record on to hard disk drives, rather than videotape.
He said security on the network had been improved by increasing the number of British Transport Police officers from 450 to 650. Sniffer dogs were also being used to check for explosives.
Mr Brown added that hundreds of staff were being moved out of ticket offices and on to platforms, where they could more easily spot suspicious behaviour.
This is partly as a result of the new Oystercard system, which has removed the need for paper tickets.
* "MILLIMETRE WAVE" SCANNER: Currently in use at Terminal Four in Heathrow and to be tested on Heathrow Express rail link. Thought to be of use only on routes with "closed access". Can detect concealed material.
* "SMART" CCTV: Can alert security staff to suspicious behaviour such as a bag left unattended. Notorious for false alarms in trials, but new equipment being tested on the New York Subway.
* X-RAY: Screening machines similar to those at airports for examining the inside of luggage. Difficult to use on railway.
* HAND-HELD DEVICES: Can be used for detecting explosives.
* BODY SCANNER: Airport-style equipment only of limited use because of the delays it would cause.
* SNIFFER DOGS: More animals under training to detect explosives.
* BAGGAGE RECONCILIATION: Staff asking passengers to account for any suspicious luggage.Reuse content