A renewed effort to remind passengers of potential terrorist situations is being launched in London by the British Transport Police.
Commuters are urged to contact police if they notice left bags
Posters in Tube stations, which see more than 3m passengers a day, will urge people to keep an eye out for unattended bags.
If the answer to "who owns this bag?" is no-one, then passengers are urged to contact the police or station staff.
The campaign is not a specific response to the Madrid train bombs, police say.
They say security is kept under constant review and there has been no specific upgrading of precautions since last Thursday when several bombs exploded on commuter trains in Madrid, killing 200 people and injuring hundreds.
The Spanish government has blamed Basque separatist group Eta for the attacks, but on Saturday a videotape was found which suggests al-Qaeda was involved.
The Lord Chancellor, Lord Falconer, reiterated Prime Minister Tony Blair's
warning that Britain would continue the fight against terror.
"We are profoundly threatened by terrorism at the moment and we can't ignore
it," he told BBC1's Breakfast with Frost programme.
"We have got to stand up to it. We have got to be clear that we are engaged
in a fight against terrorism."
The UK's rail regulator emphasised that railway stations and transport links generally in Britain were "vulnerable".
Tom Winsor, in an interview for GMTV's Sunday Programme, said: "I think the British Transport Police and the other security services, together with Network Rail and the train operators, do take every possible step to ensure that there is constant vigilance through closed-circuit TV.
"They have taken away litter bins, the surface on which any explosive devices
could be left are sloping so they will fall off - things of that kind.
The Madrid bombs were placed on commuter trains at rush hour
"It's very, very important that we maintain constant vigilance. But nothing is going
to be 100% safe."
He added that it would be "completely impractical and really unaffordable" to bring railway safety to the same level as airport safety.
"Surface land transport is vulnerable because you don't need to put the explosive device in a railway station or on a train.
"It could be on the track at any part of the national railway network.
"But the maximum steps are [being] taken to protect the railway against these
kinds of attacks and I believe that if more could be done it would be done."
The Spanish national anthem has been played at Buckingham Palace in tribute to the victims of the Madrid bombings.