Definition

noun

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.
“Stay focused on the story; you keep getting lost in all of these little sidetracks.”

verb

To divert (a locomotive or train) on to a lesser used track in order to allow other trains to pass.
To divert or distract (someone) from a main issue or course of action with an alternate or less relevant topic or activity; or, to use deliberate trickery or sly wordplay when talking to (a person) in order to avoid discussion of a subject.
“I hope you can sidetrack the teacher with questions so we don't have to take the exam.”
To sideline; to push aside; to divert or distract from, reducing (something) to a secondary or subordinate position.
“He has sidetracked this debate for far too long.”
To deviate briefly from the topic at hand.
“Just to sidetrack a little bit from the subject I will explain my reasoning.”