Mainline commuters suffered major delays and cancellations after a power supply problem in the Bedford area crippled busy routes throughout the morning peak.
First Capital Connect (FCC), Southeastern and East Midlands Trains all suffered delays of up to an hour and a half.
Only a "very limited" train service was able to operate between Bedford and Luton with passengers also having to use emergency bus services.
Trains which did run were forced to a crawl as services stacked up behind one another.
More buses operated between Bedford and Sandy to connect with shuttle train services into King's Cross.
The problem affected all FCC through-London Thameslink services services between Bedford and Brighton.
Passengers were advised to use other routes where their tickets would be accepted.
The fault, which affected all signals in the area, was discovered around 2.30 am. Network Rail engineers restored power and signals by 7.30 am but major knock-on delays continued.
