"A Gryffindor always keeps a promise, especially to someone they love." When Hermione is badly injured in a Auror raid, Sirius can only pray she is strong enough to keep her promise to protect him from the darkness of his past.
After Harry talks to Dumbledore in Heaven/Kings Cross, he knows he has to go back for final battle. But could the unexpected arrival of a train and a brief encounter with the two people he longs to see most, change his mind?