As part of Ned Wynert's quests, the Carriage Hijack missions are one of the infinite activities Syndicate's got, making the game longer and more open than previous games. We have to get on a carriage, owned by the Blighters (enemy gang) - this carriage can be stopped somewhere or moving around the city on a convoy. Once stolen, we must deliver it to a Rooks HQ.
*Imgur Album: http://imgur.com/a/7xsJw