sidetrack

A second, relatively short length of track just to the side of a railroad track, joined to the main track by switches at one or both ends, used either for unloading freight, or to allow two trains on a same track to meet (opposite directions) or pass (same direction); a railroad siding.

(sometimes) Any auxiliary railroad track, as differentiated from a siding, that runs adjacent to the main track.

A smaller tunnel or well drilled as an auxiliary off a main tunnel or well.

An alternate train of thought, issue, topic, or activity, that is a deviation or distraction from the topic at hand or central activity, and secondary or subordinate in importance or effectiveness.