The ship develops an intelligence of its own.
The Enterprise is struck by an unusual energy surge, and the holodeck begins to run on its own, creating scenes from the Orient Express and other stories. The ship itself appears to be developing a new form of intelligence, and the crew has to decide how to respond. The new being is growing out of the ship's systems.
The story is a gentle, strange episode about the birth of a new form of life, and it uses a train as an unusual setting. The crew helps the new intelligence on its way. The result is a thoughtful, whimsical story that shows the Enterprise as a place where new life can emerge. It is a unique story that serves as a coda for the series.