Where are Jack and Tobias going in Chapters 62-63? What exchange of secrets takes place on the journey?

Jack and Tobias take a train ride to Gloucester, where Tobias is to take Jack to meet the famous Thief-taker. Along the ride, Jack asks Tobias to share a secret so that Jack’s secrets can be perceived as safe. Tobias relates the truth of his affair with Lizzie and Lizzie’s pregnancy.