The Train Wreck was the event that brought about the deaths of the visitors of Narnia.
The participants were gathering together to speak of their times in Narnia. They then saw a vision of King Tirian in Narnia asking for their help. Peter and Edmund went to find the magical rings that had brought Polly and Digory to Narnia. When they had found them, they contacted the others, who took the train to join the brothers. The wreck occured as the train was coming into the station. The Pevensie brothers were at the station when they got killed by the train, while the other five Friends of Narnia were inside the train. The kids do not realize that they died until they are already in Aslan's Country. All the children who have visited Narnia and still believe in it died in this wreck, therefore excluding Susan Pevensie.
Diggory Kirke - Digory was one of the first two children to visit Narnia. He witnessed the creation of Narnia with his friend Polly. He was the one who brought Jadis the White Witch into Narnia. He was an old man by the time he was a victim of the train wreck.
Polly Plummer - Polly was the first child to visit Narnia. She also created Narnia with her friend Digory.
Edmund Pevensie - Edmund was one of the second explorers of Narnia with his other siblings. After siding with the White Witch, he came back to the good side and became a King. He visited Narnia three times.
Lucy Pevensie - Lucy was one of the second explorers of Narnia with her other siblings, and was the first of the Pevensies to enter Narnia. She visited Narnia five times.
Peter Pevensie - Peter was one of the second explorers of Narnia with his other siblings. He was the High King of Narnia. He visited Narnia twice with his siblings.
Eustace Scrubb - Eustace was in The Last Few Days of Narnia. He first entered Narnia with Edmund and Lucy, his cousins. He visited Narnia three times.
Jill Pole - Jill was in The Last Few Days of Narnia. She was the last child to be introduced into Narnia, and she visited twice.
Once the group died they were immediately transported to Aslan's Country, a sort of heaven. Since they were of great importance to Narnia and Aslan they earned their spot in his country.