Well, when grown-ups were little kids like you, they used to play with toy trains that ran on tracks, just like the real ones.
As they grew up, some of them started being very interested in these toy trains and wanted to learn more about real trains from the past. They began collecting old models of trains, tracks and other railroad-related things. They also looked at old pictures and read books and magazines about the history of trains and railways.
Now, these grown-ups form a group called the Historical Model Railway Society. They meet regularly to talk about their collections and to show them off to each other. They also recreate scenes from the past using these model trains, trying to make everything look as it did many years ago.
The Society helps people learn about the history of trains and gets them interested in preserving something that was important to many people for a long time.