The primary characters in Edith Nesbit's The Railway Children are three siblings. The oldest is Roberta (or “Bobbie”). She turns twelve during the time of the story, and she is a resourceful, brave young lady who cares very much about her family and is willing to take risks to help people in trouble. She brings Peter's broken toy train to the railway workers to fix, for instance, because she knows how important it is to her brother. She also stands courageously on the railroad tracks to stop a train heading toward a damaged stretch of tracks and therefore prevents a serious, even deadly, accident.

The second sibling is Peter. Peter is ten years old, and he considers himself the leader of his family, especially after the children's father is arrested. He is an intelligent boy, but he can be rather insensitive toward his sisters at times. Peter learns a hard lesson when he steals coal from the station and is caught. The Station Master explains to him why stealing is wrong, and Peter repents of what he has done.

Phyllis is the youngest sibling at eight years old. She tags along with her older siblings and helps as she can, especially by passing a letter to the Old Gentleman on the train who eventually becomes the children's good friend and the benefactor of the entire family.