Police have released an image of a bike which was thrown directly at a train near Enfield Lock station, bringing down overhead power lines.
The bike was thrown from a bridge on Monday, 30 December between Standard Road and Bradley Road around 5.50pm. Detectives have released pictures of the bike to try and find those responsible.
Detective Constable Paul Cudby, the investigating officer, said: "We believe the cycle was deliberately thrown and dropped on to the train as passed through... Trains travelling at high speeds risk being damaged or derailed, putting passengers' and rail staff's lives in danger."
Anyone with information is asked to call police on 0800 40 50 40 quoting incident 296 of 30/12/2013.