noun

• (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.

verb

• (rail transport) To divert (a locomotive or train) on to a lesser used track in order to allow other trains to pass.
  "It was just after 5 p.m.; would the non-stop "Capitals Limited" come up next? It was due in at 5.30 and other trains are often side-tracked in its favour."
• (intransitive) To deviate briefly from the topic at hand.
  "Just to sidetrack a little bit from the subject I will explain my reasoning."