Which Assassin's Creed game introduced the train-based headquarters called the Rooks' train?

Assassin's Creed Syndicate introduced the Rooks' train as a mobile headquarters for Jacob and Evie Frye.

The train is obtained early in the London campaign and serves as a base that moves through the city. It provides access to upgrades, character management, and other functions while matching the game's Victorian setting. The mobile headquarters also reflects the importance of rail transport during London's Industrial Revolution.

The Rooks are Jacob and Evie's allied street gang. As the twins liberate London districts and recruit allies, the train becomes a recognizable symbol of their operation. Its movement and design distinguish Syndicate from earlier games, whose headquarters were generally fixed buildings or settlements.

The train should not be confused with the Jackdaw, Edward Kenway's ship in Assassin's Creed IV: Black Flag, or the Morrigan, Shay Cormac's ship in Assassin's Creed Rogue. Those vessels are both transportation and gameplay platforms, but the Rooks' train is specifically associated with Victorian London.