The Conductor is a holodeck character aboard the Orient Express in Emergence, who keeps the train and its odd mix of passengers moving towards the destination chosen by the Enterprise-D's emerging intelligence.
The Conductor appears in the seventh-season episode Emergence. When the ship begins acting on its own, the crew find that several holodeck programmes have merged into a single scenario set on the Orient Express, crowded with a knight, a gunslinger, a flapper, gangsters and others.
The Conductor moves through the carriages checking tickets and insisting that the train will keep to its route. He treats the Enterprise officers as passengers without proper tickets and resists their attempts to stop or redirect the train, at one point having them put off.
Each character stands for a part of the ship's developing mind, and the Conductor represents its drive to reach its goal, the creation of a new life-form, which the crew eventually decide to help rather than hinder.