What's the noun for sidetracks? Here's the word you're looking for.
sidetrack
(rail transport) 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.
(mining) A smaller tunnel or well drilled as an auxiliary off a main tunnel or well.