Commuters escaped unhurt when a gang of youths threw bricks and stones at passing Tube trains in west London.
The group targeted several trains on the Central Line in the Acton area on Wednesday night.
Several windows were smashed and the trains affected had to be taken out of service as a result.
British Transport Police (BTP) are investigating the "reckless vandalism", which follows a number of previous attacks on the same line.
Insp Michael McGinty said: "This was a reckless act of vandalism and I cannot emphasise strongly enough the dangers associated with this type of crime."
Police want any witnesses to the attacks to get in touch.
A spokesman for Transport for London (TfL) said on Thursday: "A number of Central Line trains were damaged by vandals yesterday evening in the Acton area.
"Several train windows were broken, resulting in the trains being taken out of service. There were no injuries to passengers or staff."