What does the joke "Send a friend to the high-speed rail station, and then go buy oranges as soon as the train starts" mean?

This paragraph is adapted from Zhu Ziqing's "Back View". In the original text, Zhu Ziqing's father bought some oranges for Zhu Ziqing before driving.

Zhu Ziqing (November 22, 1898 - August 12, 1948), whose original name was Zihua, was named Peixian and Qiushi. Originally from Shaoxing, Zhejiang, he was born in Donghai, Jiangsu, and grew up in Yangzhou, Jiangsu, so he said "I am a Yangzhou native". Graduated from Peking University and served as professor and director of the Chinese Department of Tsinghua University. Chinese modern poet and prose writer. The writing style is fresh and co-edited into the complete works of Zhu Ziqing. It adds a magnificent color to modern Chinese prose, and creates a prose system and style with Chinese national characteristics to establish a new aesthetic feature of modern Chinese prose.

"Back" describes the process of a father bidding farewell to his son who is traveling far away when the family suffers a tragedy. Through simple and sincere language, it expresses the father's love for his son and the son's longing for his father. It is a famous piece in the history of modern Chinese prose. The author once said: "I wrote "Back" because of the sentence in my father's letter quoted in the article. When I read my father's letter, I really burst into tears. My father treated me with many good things, especially " The scene narrated in "Back View" is no different when I think about it.

The sincerity of Zhu Ziqing's prose is well-known. His "Back View", "Mourning for the Dead Woman", etc. "The first-class love literature in the world". In the light pen and ink, there is a deep feeling, without any artificiality, but with touching power, especially in "Back View", Zhu Ziqing's feelings for his father Zhu Hongjun. It makes readers feel a trace of nostalgia and emotion.

"Back" is a documentary prose written by Zhu Ziqing in October 1925. "Back" describes the situation when the family suffered a change. The story of a father bidding farewell to his son for a long journey shows the father's love for his son and his son's gratitude for his father through simple and sincere language. It is a famous piece by the author in the history of modern Chinese prose. He once said: "I wrote "Back" because of the sentence in my father's letter quoted in the article. When I read my father's letter, I burst into tears. The many good things my father did to me, especially the one narrated in "Back", are the same when I think of them as they are in front of me. My article is just realistic..." This sentence tells the reason for writing "Back View", the focus of description and writing characteristics, etc., which can be used as the key to understanding the article.