A rail operator reportedly failed a security test after undercover investigators placed a fake bomb in a train carriage - and it went unnoticed.
The suspicious device was left in a bag and placed on a busy Southern Rail train, operated by GTR, as part of an anti-terror initiative test, according to The Sun.
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In a November memo obtained by The Times, Tony Holland, the crime and security manager of GTR wrote: "Last week transport security inspectors from the Department for Transport (DfT) carried out a covert test on one of our trains.
"The test involved leaving an unattended bag on a train, which contained a suspicious item.
"Disappointingly, the bag was not found despite it being positioned in a public area of the train, and railway personnel seen to walk past the location."
Southern Rail has been subject to a number of strikes in an ongoing row over the role of guards amid plans for drivers to open doors.
Speaking to The Times, a spokesman for Aslef, the train drivers' union, said: "It is quite extraordinary that Southern Railway has failed a DfT security test while getting rid of the guards on the trains. How does this help reassure passengers?"
The spokesman added: "All railway staff are acutely aware of the increased security threat and we pass the overwhelming majority of DfT's routine tests and investigate any that we fail."
A Southern Rail staff strike is planned to start on Monday.