Many trains were cancelled, delayed or re-routed this morning, thanks to signalling problems.
The fault was between Purley and Tattenham Corner, but rail operator Southern said there was an impact on all of their lines.
A number of services were affected, mainly to and from London Bridge, until late morning.
Network Rail engineers were said to have been working on the problem since 7.35am.
Passengers are advised that if they were delayed by 30 minutes or more then they may be eligible for compensation.
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