It was one of the worst rail disasters the country has ever seen. Today, 30 years on - it was remembered with a memorial service attended by survivors, the emergency services and the rail industry.
A train from Bournemouth crashed into the back of a packed commuter service from Basingstoke at 8:13am in the morning near Clapham Junction.
Another in the opposite direction crashed into the wreckage. A wiring error was blamed for causing a signal that should have been red showing as green.
Watch this very moving report by Emma Wilkinson:
For survivors there are moments of that day in 1988 that are etched on their memories. They describe how shock, confusion, adrenaline, influenced their decisions and perceptions.
For some, post-traumatic stress went on to have a major impact.
As survivors, they felt guilty.
Alex Keegan, survivor:
With an overwhelming sense of pointlessness, Alex began writing books and teaching. He now works with asylum seekers, documenting their stories.
Marilyn Robinson too changed her life.
She helped start the charity Disaster Action, which brought out a book.
Through people's experiences, it aims to advise those who respond to disasters, and help those affected by them. It's the kind of information that was lacking 30 years ago.
30 years on, as they remember those who didn't walk away, survivors are determined to live a life of meaning.