Trains between the East Midlands and the capital were severely delayed after a rodent chewed through cabling.
Disruption between stations in Nottingham, Leicester, Derby and London lasted more than 12 hours with trains delayed from the start of service until 7pm on Monday.
The chewing rodent shut down signals between Wigston and Market Harborough, Leicestershire.
A Network Rail spokesman said: "We apologise for the significant disruption to journeys yesterday which was caused by damage to a trackside cable.
"The cable, which had been damaged by a rodent, was located just north of Kilby Bridge, and lead to a loss of power to signals between Wigston North Junction and Market Harborough station, just after 4.45am, causing all trains to stop.
"Although power was successfully restored just after 11.15am, trains and crews were out of position and it took until the evening rush hour for timetabled service to be resumed fully."