A Duke in Time Saves Nine by Emily E.K. Murdoch

Byron Renwick, Duke of Sedley, is a man on a mission. Vengeance fills his soul, and he will stop at nothing to gain justice.
Which is why he finds himself chasing after a mail coach in the dead of night with a mysterious and rather beautiful passenger. It wasn’t supposed to be like this. After discovering the Glasshand Gang’s plans to rob a mail coach, Byron was determined to catch them in the act and bring them to justice.
Missing the mail coach, and accidently letting slip to a woman that everyone in the mail coach was in danger? Also, not the plan.
Anne Mead has just put her darling younger sister on that mail coach and won’t take no for an answer from the strange gentleman who seems to know the nine people on that coach are now risking their lives. She tells him in no uncertain terms: Anne is going with him. Together, they’ll save her sister.