Prompt action by rail staff averted a possible major disaster when two huge construction drills came through a tunnel wall on a major commuter route.
The incident happened after a driver of Friday's 10.02am First Capital Connect (FCC) train from Moorgate in London to Welwyn Garden City in Hertfordshire saw muddy water pouring on to the roof of his train north of Old Street station.
He reported the incident and all rail services were stopped.
At 11.05am a Network Rail (NR) mobile operations manager joined another driver on board a special empty train from Moorgate to investigate the problem.
As the train approached the area at low speed, two large piling drills - used to thump and screw their way in to the ground - came through the skin of the tunnel of the Moorgate to Finsbury Park Northern City line. The train stopped in time but the driver was shaken.
The rail route, used by two million City workers every year, will be closed for the rest of the day and possibly longer while the incident is investigated and repairs are carried out.
British Transport Police have visited an office construction site above the line and the Rail Accident Investigation Branch has been notified.
FCC managing director Neal Lawson said: "This was 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.
"Meanwhile, we have published details of alternative travel arrangements and are urging our passengers to check www.firstcapitalconnect.co.uk for the latest news but the line will be out of action for the rest of the day at least."