Armed police officers are patrolling on board trains nationwide for the first time, British Transport Police (BTP) has confirmed.
The measure has been introduced in a bid to "disrupt and deter criminal activity" on the rail network after the UK terror threat level rose to critical in the wake of the Manchester attack.
Armed officers have been patrolling on the London Underground network since December, but this is the first time they will travel on trains outside the capital.
BTP urged passengers not to be alarmed by the presence of firearms officers.
Chief Constable Paul Crowther said: "Since the devastating events in Manchester on Monday evening, our force has radically increased the presence of our officers nationwide.
"By having firearms officers on board trains we're ensuring that trains remain as safe as possible for passengers.
"Our patrols will be highly visible and passengers should feel comforted by their presence.
"Please do speak to them if you have any concerns at all," he added.
Mr Crowther went on: "I want to take this opportunity to remind everyone to stay calm, be vigilant, and if you spot anything at all which gives you cause for concern, let us know.
"The cowardly acts of terrorists will never weaken our resolve."