"Safety is our top priority and the cause of this incident is being investigated. We apologise to customers for the inconvenience caused by the power cut and our staff restored all supplies as quickly and safely as possible."
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The three-week trial is designed to speed-up the flow of passengers leaving Holborn station during the morning rush hour.
Tube managers unveiled plans last night to hire a new workforce of part-time drivers in a bid to rescue the 24-hour service.
Masked machete-wielding raiders stormed into a convenience store and threatened the assistant, before stealing cash, cigarettes and alcohol.