The diesel locomotive, known as "Thunderbird", contains British Transport police officers and Railtrack staff.

The project also aims to use police helicopters fitted with heat-seeking cameras.

The train is operating as part of Railway Crime Week and hopes to reduce the £250m estimated annual cost of vandalism.

Officials unveiled the scheme at Exeter St David's station on Friday.

Railtrack spokesman Christian Tagholm said the strength of the idea was in its flexibility: "This can be used both reactively and proactively.

"We can send it quickly to any part of the south West network in response to an incident.

"Then again we can use it as a deterrent, patrolling the network.

"If necessary those on board can pursue and detain offenders."

British Transport Police estimate 50% of damage to rolling stock is caused by deliberate action.

Incidents in the south west include a shopping trolley being left on the line at Teignmouth and stones being thrown at trains in Exeter and Plymouth.