Train passengers faced delays, diversions and disruption as signal failures and other problems plagued the main line between Brighton and London.
There were no trains at all between Brighton and Three Bridges – and the Gatwick Express service was suspended.
The west coastway service was also affected with no trains to or from Southampton.
Train operator Govia Thameslink Raolway (GTR) said that this was because of “a number of incidents”.
Commuters returning to Brighton were diverted via Lewes or Horsham, adding at least an hour to the duration.
GTR said that services to and from Brighton and Cambridge had also been suspended.
At Wivelsfield, another signalling problem meant that drivers were having to be talked through the signals there.
GTR also warned of delays facing anyone travelling to Gatwick and urged passengers to allow more time to reach the airport.
The disruption was expected to last until the end of the evening, with Southern, which is part of GTR, advising people not to travel by train for the rest of the day.
