Training Rooks II is the second training tier, pushing your gang to level 7 so they hold up in mid-game boroughs. It is a big jump in price from the first tier.
Training Rooks II is entry 3 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: £10,000, 100 Metal, 50 Silk. Before Training Rooks II unlocks you need Rook Training I and Allies in Arms.