I think Sassoon chose to end the poem with the image of a man going to an office by train because it represents the mundane reality and routine of daily life. By contrast, the dreamers in the poem are portrayed as imaginative and disconnected from the constraints of ordinary life. The final image serves as a reminder of the responsibilities and obligations that ultimately pull us back from the freedom of our dreams. It highlights the tension between the dream world and the real world, emphasizing the struggle to balance the two. Overall, the ending image adds a sense of closure and brings the reader back to the present moment after exploring the realm of dreams.
