Transport Secretary Justine Greening has said the £9billion planned investment in the railways will lead to a "better and more cost effective" service for passengers.
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The Information Commissioner’s Office said BT did not have customers’ consent to send the emails.
The 23-year-old was held on suspicion of doing an act likely to cause an explosion to injure or damage property, and endangering safety.
Three men died on the track between Brixton and Denmark Hill in south London.