* Introduction of a mystery: The children's encounter with the "gentleman" raises questions about his identity and his presence in the tunnel. This creates suspense and sets the stage for the central mystery of the book.
* Development of the children's curiosity and sense of adventure: The discovery ignites the children's imagination and encourages them to explore the world around them. They start to see the railway not just as a means of transport but as a place of hidden secrets and exciting possibilities.
* Foundation for future events: This encounter directly leads to the children's continued involvement with the "gentleman" and their eventual efforts to help him, culminating in their involvement in the central plot of the story.
Overall, Chapter 6 is a pivotal moment in The Railway Children as it introduces the central mystery, ignites the children's curiosity, and sets the stage for the main plot of the story.