After what he saw at Borgin and Burkes, Harry is pretty much convinced that Malfoy is a Death Eater working for Voldemort. He thinks that Draco showed Borgin the Dark Mark on his left arm, and that's also why Draco jumped when Madam Malkin touched his arm.
Ron, Ginny, Hermione, and Harry board the Hogwarts Express, and Harry finds that he is stared at and whispered about everywhere he goes. The fame thing has never been this bad before. Since the incident at the Ministry of Magic last year (in Book 5), the Daily Prophet has been speculating whether Harry is the Chosen One, and everybody is intrigued.
Harry rides in a compartment with Luna Lovegood and Neville Longbottom, even though he is invited by a group of admiring girls to sit in their compartment.
Slughorn sends Harry an invitation to come to a little get-together in his compartment.
Harry nearly dies of boredom and of frustration at Slughorn's gathering. It would seem Slughorn has gathered all of the students with rich or powerful family connections. Ginny Weasley is the only exception – she impressed Slughorn by casing a really great hex on an annoying boy.
Slughorn pesters Harry with a bunch of questions about the incident at the Ministry, and whether or not Harry really is the Chosen One. But Harry does his best to avoid answering any of the prying questions.
On his way out of Slughorn's compartment, Harry uses his Invisibility Cloak to sneak into Malfoy's compartment. After almost getting caught, he crams himself into the luggage rack.
Harry overhears Draco discussing Voldemort and suggesting that he might be working for the Dark Lord.
Inadvertently, Harry lets out a little scream when the train approaches the Hogsmeade station. No one seems to notice.
When the train stops, everyone clears the compartment except for Malfoy, who knows Harry is there.
Malfoy freezes Harry, who falls to the ground, and then stomps on Harry's nose. He leaves Harry, still covered in his Invisibility Cloak.
Harry can't move and is filled with hatred for Malfoy.