Rail services on some of the country's busiest routes were disrupted on Wednesday because of a number of signalling problems.
South Western Railway services, including those to and from London Waterloo, the UK's busiest railway station, were delayed or cancelled.
The delays were caused by signal problems at Havant, which blocked lines.
Disruption is expected until the end of the day.
Passengers were told that some trains will be changed or cancelled in the morning, while other changes will be made at short notice.
Southern Rail and Great Western Railway trains were also subject to change, potentially at short notice.
- The Rail, Maritime and Transport union confirmed that its members on SWR will take industrial action over the Easter weekend by refusing to work rest days in the long-running dispute over the role of guards.