Commuters travelling to London have faced train delays and cancellations for a second day.
Tens of thousands of passengers faced "nightmare" overcrowding and long delays on Thursday due to a signalling fault in Streatham, south London.
Passengers were advised to travel as normal on Friday, but were warned services could be cancelled or delayed.
Engineers are yet to work out why a panel providing power to signalling equipment "burned out" on Wednesday.
Govia Thameslink Railway, (GTR), which operates rail firms Southern, Gatwick Express and Thameslink, said several Southern services into London Victoria were cancelled on Friday morning as a knock-on effect of Thursday's chaos.
Some Southern trains also ran with fewer carriages than normal, while there were minor disruptions to Thameslink services, a spokesman said.
He said Gatwick Express trains ran a good service.
On Thursday many passengers with tickets to or from London Victoria were advised not to travel to the capital at all.
Victoria is the second busiest station in Great Britain.
About 210,000 passengers use the station each day, according to average figures collected for the Office of Rail and Road (ORR) in 2016-17.
The signalling system, which went down when a power supply failed in the Streatham Common area on Wednesday night, was "back in correct working order", said National Rail.
But it had warned there would be a knock-on effect on Friday morning.
Train carriages were "not in the correct places", and matters had been "complicated" by the displacement of crew members.
'Failed in job'
On Thursday, commuters complained of being "sardined" into overcrowded trains, while others missed flights from Gatwick.
The airport told customers due to fly on Friday to leave extra time for their journey to the terminal.
John Halsall, managing director of Network Rail's South East route, apologised for Thursday's chaos and said the company had failed in its job to provide "a reliable railway for train operators and the travelling public".
On Friday, a Network Rail spokesman said the signalling equipment failed after a switching panel designed to provide back-up power was found burned out, and the room it was in was "full of smoke".
He said: "Our team worked through the night to bypass it, and had the signalling system working again by mid-morning.
"Unfortunately, that fix then failed before we could run trains and a new design had to be found. That is why it took so long."
Network Rail said it was now investigating to prevent similar faults happening again.
Southern encouraged people whose trains are delayed by 15 minutes or more to claim compensation.
The disruptions come as GTR is set to launch a special compensation scheme for people severely affected by a chaotic timetable change in May.