London's giant Crossrail project has reached a 'worrying stage' following an unexpected electrical explosion, according to a senior City Hall official.
Underground test runs of the new £1bn train fleet were halted after two electrical transformers blew up at a sub station in east London.
The setback, blamed on a 'design error' is preventing engineers from switching on the overhead cables which power the trains.
A fresh attempt last night to 'energise' the system failed. The explosion on November 11th was only revealed by Crossrail bosses this week.
Deputy Mayor for Transport Val Shawcross today admitted the £14.8bn project faced a number of risks, including possible problems with the complex signal software.
Crossrail, renamed the Elizabeth Line is due to open in December.