Railway lines between London Euston and Watford Junction have reopened as emergency services responded to a “casualty on the track” near Bushey.
London Overground trains are part-suspended while Avanti West Coast and London Northwestern Railway services are also disrupted.
National Rail says the “major” disruption is expected to last until the end of the day.
Meanwhile trains are running through Barnes station once again after a knife attack left a man fighting for his life.
The station was shut following the stabbing of a man in his 20s, who has been rushed to hospital in a life-threatening condition.
It comes as the M25 in Essex was closed anti-clockwise between J28 near Brentwood and J27 following a crash between a lorry and a van. Two people have been rushed to hospital following the crash, which caused a massive 20-mile tailback, according to traffic monitoring site Inrix.
