Tris, Christina, and Tobias are grabbing onto the train in order to make their way to the outside world. The only way to get inside the train is to run on to the platform and grab the handlebars.
Tris is trying to think of the possiblities of going to the new world. She becomes a little nervous because she does not know how to people will react when they see her. It is important because if the group did not travel to the unknown place, then they would never be able to see what kinds of people are living outside their world.
The gang is viewing the unknown city after getting off of the train. This scene is significant because the members want to find out what lies beyond their world. This is highly dangerous for them and could possibly lead to death. This scene was also intriguing because the author describes the city as never seen before in the characters eyes, but in reality, she is describing the modern day city of Chicago. The book was set in the dystopian era, so it is interesting to imagine what life could be like in the future.