What is the fine-grained topic of the following text: He watches it, he waits for it, he plays nears its tracks, he listens for the weight of its currents (strong currents flowing east pulling two hundred boxcars, light current going west with less than fifty cars), he whiles away long summer days throwing rocks and bricks at it with his friends Danny, Marco and Little. Only return the topic.
A boy's fascination with trains.