South West Trains say a problem at a level crossing between Barnes and Richmond means all lines will be blocked and services disrupted until around 6.30pm.
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Tributes have been left in Caledonian Road in Islington where 15-year-old Alan Cartwright died after he was stabbed.
A boy has died after he and his friends were attacked while cycling in north London.
Three police officers have been praised for their actions in helping a four-year-old boy who had become stuck on a roof in Newham.