Network Rail engineers were carrying out emergency pumping operations in a flooded rail tunnel in north London on Friday afternoon.
Water was gushing into Clerkenwell Tunnel from a burst 400mm-diameter water main on the street above.
Staff with temporary pumps were deployed by Network Rail to keep the water level in the tunnel below the height of the rail head.
This was allowing trains to pass through between Farringdon and St Pancras stations.
However, a limited service was running on Friday as the water has knocked out the signalling system, requiring a different method to be used to control train movements.
Thames Water was dealing with the burst water main, and Network Rail had no timescale for returning to a full service in the tunnel.