Rail experts were still investigating on Monday how overhead power lines came to collapse across all four tracks of Britain’s busiest main line on Sunday, causing serious disruption.

The power lines on the London-Glasgow west coast main line north of Watford Junction were brought down when the pantograph of a Virgin Pendolino tilting train became caught in the 25,000-volt cables. One railway industry observer, who asked not to be named, said the damage was extensive because it occurred near a junction where overhead lines on different tracks were connected.