On the Hogwarts Express, Rose tells Albus she wants to be friends again.
He reminds her, "We never were friends" (1.10.6).
Is there a spell to heal that sick burn?
Even though Albus had listened in a few scenes ago when Amos Diggory said Harry had a Time-Turner, Albus is shocked to learn from Rose that Harry has a Time-Turner.
That gives Albus an idea.
He finds Scorpius and tells him that they have to get off the Hogwarts Express (while it's in express mode).
Why? Because Albus thinks if he can bring Cedric Diggory back, he can correct his dad's past mistakes.