Concrete piece thrown at Metropolitan Line Tube train
A piece of concrete was thrown at a Metropolitan Line train from a bridge in north-west London, police said.
The fist-sized piece was thrown at the Tube train on 29 February from Highfield Road Bridge as the driver was pulling into Northwood station.
The concrete piece dented the front of the train, but no-one was injured, the British Transport Police (BTP) said.
BTP officers are also investigating the theft of railway cable on Jubilee Line from Queensbury early on Monday.
The incident was reported in the early hours. No arrests have been made.
Speaking about the concrete incident, the BTP spokesman said: "Throwing stones at trains is an incredibly irresponsible thing to do.
"This was an extremely foolish and dangerous act and it was lucky no-one was injured.
"We urge those who think that throwing stones at trains is fun to think about the consequences of their actions, which in this case could have been much worse."
There were no passengers on the northbound train at the time of the incident. No-one has been arrested.