Quick action from rail staff prevented a major incident in London on Friday when what appeared to be two large piling drills came through the tunnel wall of the Moorgate to Finsbury Park Northern City Line.
The rail route, used by two million City workers every year, was closed for the rest of the day while the incident was investigated and repairs were carried out.
First Capital Connect praised its train drivers for their vigilance in averting a major incident.
The train operator said at 10.09am the driver of the 10.02 Moorgate to Welwyn Garden City service with passengers on board saw muddy water pouring onto the roof of the train north of Old Street station.
He immediately reported the incident and all rail services were stopped.
A Network Rail Mobile Operations Manager later joined another driver on board an empty train from Moorgate to investigate the problem and as they approached the area at low speed, two large piling drills came through the skin of the tunnel.
The train stopped in time but the driver was shaken.
First Capital Connect's 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.
Phil Verster, Network Rail route managing director, said: 'Our engineers are on site assessing the damage and we will restore the railway as soon as possible.'
The line is normally closed at the weekend and engineers are working to reopen it in time for Monday's morning service.
Image: First Capital Connect
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