Commuters can become "oblivious to their surroundings" and miss potential signs of extremist behaviour, transport policing chiefs have warned as a nationwide anti-terrorism campaign continues.
Paul Crowther, chief constable of the British Transport Police (BTP), said passengers using public transport should be alert to the threat of terrorism.
His comments came as national counter-terrorism awareness week, which will see some 6,000 people briefed by officers at 80 venues across the country, continued.
Yesterday, the Home Secretary unveiled a sweeping package of counter-terror measures as it moved to bolster the UK's defences amid warnings of a growing extremist threat that is set to last for several years.
As the counter-terror campaign shifts focuses to protecting transport hubs, Mr Crowther said: "More than six million people travel on our railways every single day.
"For commuters, who make the same journey over and over again, it can be easy to become oblivious to their surroundings.
"But I would urge them to remain alert, use their instinct and have the confidence to report anything that strikes them as out-of-place or suspicious."
Mr Crowther highlighted the case of Andreas Pierides, who was given an 18-month suspended sentence after admitting taking eight explosive flares on to a plane at a UK airport and possessing a terrorist manual.
The alarm was raised when a passenger, looking through a gap in a train seat, noticed Pierides was researching explosives and had instructions about how to make a bomb.
She instantly reported her concerns and police used CCTV to track him to the station where he bought his ticket. He was subsequently identified and arrested at Stansted Airport attempting to fly out of the country.
"This was as a direct result of a rail passenger reporting suspicions to train staff," Mr Crowther said. "We need others to follow suit and play their part in keeping the UK's transport systems safe from terrorists."
BTP have units across the UK, monitoring stations and trains every hour of every day.
They include units with specialist equipment to quickly assess the threat of suspect packages, officers trained in identifying suspicious behaviour and highly trained police dogs.
Police will be at transport hubs across the UK to raise awareness of the work they are doing to protect transport networks and encourage members of the public to report suspicious activity.
The Assistant Chief Constable of Merseyside Police, Andy Ward, spoke yesterday at Liverpool Docks about Operation Kraken, which encourages people who work along the UK coastline to report suspicious activity.
Several hundred people are believed to have travelled to Syria so far this year and police at ports and airports are working to stop travel for this purpose.
Counter-terrorism awareness week comes a few months after the terror threat level in the UK was raised from substantial to severe, meaning a terrorist attack is ''highly likely'', against a backdrop of increasing concerns over hundreds of aspiring British jihadis travelling to Iraq and Syria to learn terrorist ''tradecraft''.
Fears of a terrorist attack on Britain's streets have heightened in the wake of the rise of Islamic State (IS), the extremist group that has taken over large swathes of Iraq and Syria and attracted thousands of foreign jihadists to its cause, including more than 500 Britons.
Addressing the Royal United Services Institute (Rusi) in London yesterday, the Home Secretary said the threat from jihadists and other fanatics is "greater than it has ever been" as she revealed the range of draconian powers included in a new Counter-terrorism and Security Bill.
Within the new bill, which will be introduced to Parliament on Wednesday and should pass into law before the general election, is a legal requirement by schools, prisons and councils to put in place policies or programmes to stop would-be extremists from being drawn into terrorism.
Legislation will be clarified to make sure insurance companies can no longer foot the bill for terrorist ransoms, suspected foreign fighters will be blocked from returning to the UK and powers will be re-introduced to relocate terror suspects across the country.
Mick Cash, general secretary of transport union RMT, said: "The eyes and ears of the commuter transport system are the staff working on the trains, the platforms, the stations and on the buses.
"While the police alert to commuters is perfectly sound, it is ludicrous the Government and the transport companies are looking to axe guards, conductors and station staff who are trained to spot suspicious behaviour and to deal with evacuations if suspect devices are spotted on the transport system.
"Cutting those staff is a potentially lethal gamble."