Why does Tom drag Nick off the train on their first trip to New York in The Great Gatsby?

In chapter 2, Tom and Nick take a train to New York. On their way to New York, they pass a depressing, impoverished place known as the "valley of ashes." This is where Tom's current mistress (Myrtle Wilson) lives with her husband. When the train stops at the aforementioned valley, Tom "jump[s] to his feet and ... force[s]" Nick from the train. Tom says to Nick, "We're getting off!" and then tells him, "I want you to meet my girl." Tom's forcefulness here is in part because he "tanked up a good deal at luncheon," meaning that he had drunk a lot of alcohol at lunch. He is also simply excited at the prospect of soon meeting up again with his mistress to continue their illicit affair.

One reason Tom is so insistent that Nick should meet Myrtle is simply because he wants to show Myrtle off to Nick. Myrtle is described as a sensuous woman with a body that is "continually smouldering." She is a prize that Tom likes to show off to boost his own ego and to validate his own sense of masculinity. Tom takes great pride in his masculinity, which he seems to validate by having lots of affairs with women.

Tom takes Nick to Myrtle's husband's garage, to arrange a rendezvous with Myrtle. While at the garage, Tom also seems to take great pleasure in humiliating Myrtle's husband, George. He is brazenly dismissive of George and tells Nick that George is "so dumb he doesn't know he's alive." Humiliating George in front of his wife and in front of Nick is another way in which Tom likes to show off his supposed masculinity.

What are Nick's thoughts about getting off the train with Tom in The Great Gatsby?

Nick's thoughts about leaving the train with Tom are that Tom's sudden insistence that Nick get off with him at the Valley of Ashes "bordered on violence."

Nick states that Tom took his elbow and "literally forced" him from the train car, telling Nick that he wanted him to meet Myrtle, his lover. Nick adds that Tom seemed to be "tanked up" or tipsy from lunch and that he arrogantly assumed that Nick had nothing better to do on a Sunday afternoon than spend it however Tom wanted. (Of course, Tom might have been right, as Nick does unprotestingly spend it with Tom and Myrtle.)

Tom's action is consistent with Nick's general perception that Tom is a brutal, arrogant person who expects people to do what he wants and isn't subtle about imposing his will. As Nick goes with Tom to the Wilson's garage and gas station, he witnesses more of Tom's bad behavior. Tom is stringing George Wilson along about selling him a car, as it gives him an excuse to visit the garage. He speaks "coldly" to George and later speaks of him to Nick as a complete nonentity who has no idea that his wife is cheating on him.

Nick also has the opportunity to meet Myrtle, an energetic, sexual woman who to a large extent uses the relationship with Tom to make small purchases that he can easily afford.