Data 73 was a holodeck program created by Data. It was a simulation of Act 5, Scene 1 from William Shakespeare's play The Tempest. This scene was set in a room, on Prospero's Island, where Prospero practiced his Neoplatonic rites.
In 2370, Data used this program for improving his performance as Prospero. While being given insight on this character by Jean-Luc Picard, there was a malfunction in the program, where two programs temporarily linked. The Orient Express thundered through the room, narrowily missing Picard and Data. This train was from a program created by Doctor Beverly Crusher. (TNG: "Emergence")