When the train stopped at the river in "The Great Gatsby", Tom Buchanan got off with Nick. Why?
Tom and Nick stopped to go into the Wilson's garage. Tom's mistress, Myrtle, is married to Mr. Wilson who owns the garage. Tom wants to let her know that they are going to the city so that she would follow along. He pretends to Wilson that he stopped in to say hello, and the two chat about Tom's car, which Tom has suggested he might sell to Wilson. The conversation is just a front, of course, for Tom to arrange to have Myrtle take the next train to NYC.
This is the second chapter, and we see how quickly Nick is being drawn into the world of corruption. He is already a witness to infidelity, and he will soon be drinking with shallow people he doesn't know. Besides introducing to the pervasive corruption of the society, Fitzgerald is also reinforcing Nick's declarations about himself. Nick has said that he doesn't judge others, and that is has caused many men to confide in Nick:
In consequence, I’m inclined to reserve all judgments, a habit that has opened up many curious natures to me and also made me the victim of not a few veteran bores.
Fitzgerald lets us know that this is true; Tom is already trusting Nick to engage in this encounter, knowing that Nick is Daisy's wife. We would assume Tom to more discreet. But he isn't, suggesting his own arrogance, but also Nick's open mind. He doesn't stand up against Tom, after all, and complain that Tom is being unfaithful to Nick's own cousin.