A holodeck recreation of the historic European luxury train, run by Beverly Crusher, which became the core of the fantasy through which the Enterprise's emergent intelligence expressed itself.
The Orient Express appears in the Next Generation episode Emergence. Its first intrusion comes when Data is rehearsing a scene from The Tempest for Picard and a locomotive bursts out of the darkness across the stage, nearly running the captain down.
Crusher had been using the programme for relaxation. The crew then find that it has fused with several others, so its carriages are filled with a hitman, a flapper, a knight in armour, a gunslinger and a nervous conductor, all cooperating on a journey to Vertiform City and hostile to interference.
Because the safety systems are off, away teams must play along. Troi, Data and Worf ride the train to understand what the ship wants, and Worf even shovels coal to keep it moving.