The Rail Maritime and Transport Union said that it was balloting drivers on the Picadilly line in a row over working conditions and agreements.
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Anthony Birdee from Deptford in south east London tied the English bull terrier, named Muller, to a post in his garden.
A report warned overcrowded streets could lead to a 'Hillsborough scale' tragedy.
Southern got just one star for punctuality, reliability, seat availability, frequency and value for money.