Armed teams of British Transport Police (BTP) are to patrol the railways and London Underground to counter the terrorist threat, the Government has announced.
Until now BTP officers have not carried weapons but Transport Secretary Philip Hammond has said the force would now have its own armed capability.
He said armed BTP officers would be "deployed as appropriate in response to the terrorism threat level at any given time".
Armed police from other forces sometimes patrol stations and trains.
In a Commons statement Mr Hammond said that by training BTP officers to carry out armed patrolling of the rail network it "equipped them with a capability already available to other forces".
He added that it would not be a daily event to see armed officers at stations and they would be deployed "according to operational need".
Mr Hammond said: "The Government has been considering the resilience of the overall police armed capability and has concluded that it would be beneficial to enhance this by providing the BTP with an armed capability of its own.
"The timing of this is not as a result of any specific threat: it is a sensible and pragmatic approach to ensuring that our police forces have the right resources to be able to respond as and when needed to protect the public."
BTP Chief Constable Andy Trotter said: "BTP officers have an excellent working knowledge of the railway which will enable them to respond quickly to any incidents."
The 7/7 attacks on the Underground in 2005 highlighted the vulnerability of an "open" system such as the Tube, which, because of its nature, cannot become a "closed" system like an airport, where passengers can be thoroughly security-checked before passing airside.