Trains were cancelled on a busy London route last week after after two large piling drills broke through a tunnel wall on the Northern City Line.
Passengers on the 10.02 Moorgate to Welwyn Garden City service on March 8 spotted muddy water pouring on to the roof of the train just north of Old Street station.
An empty train was then sent out to investigate the incident and as it approached the area, the two drills came through the skin of the tunnel.
The British Transport Police visited an office construction site above the line and the Rail Accident Investigation Branch has been notified.
FCC managing director Neal Lawson said: “This is a serious incident that could have ended very differently had it not been for the vigilance and prompt reporting and actions of our drivers.
“We carry two million people a year on the Northern City Line and whoever is responsible for this must be held to account.”
Network Rail said specialist tunnelling contractors had been sent to the scene to assess the damage.