Rook Riders II increases the number of Rooks carriages you see around London. It is the second step toward summoning a carriage of your own.
Rook Riders II is entry 7 of 17 in the Ringleader branch, one of the three branches (Ringleader, Insider and Swindler) on the Rooks' 39-upgrade board. It is bought with money and materials, not skill points: £3,000, 100 Metal, 50 Leather. Before Rook Riders II unlocks you need Rook Riders I.