Police say anyone who witnessed the shooting, or has any information should contact detectives on 0208 358 0200, or Crimestoppers anonymously on 0800 555 111.
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Thousands of passengers now face delays, with Southeastern trains warning "all services" would be affected.
Paramedics attended a scene in Willesden, north-west London, on Monday afternoon where they found the teenager with stab wounds.
At least five people have suffered minor injuries but the cause of the blast is still unknown.