The Collaborative International Dictionary of English v.0.48:

Sidetrack \Side"track`\, v. t. [imp. & p. p. Sidetracked; p.
   pr. & vb. n. Sidetracking.]
   1. (Railroads) To transfer to a siding from a main line of
      track.
      [Webster 1913 Suppl.]

   2. Hence, fig., to divert or reduce to a position or
      condition that is relatively secondary or subordinate in
      activity, importance, effectiveness, or the like; to
      switch off; to turn aside, as from a purpose. [Colloq.]

            Such a project was, in fact, sidetracked in favor of
            the census of school children.        --Pop. Sci.
                                                  Monthly.
      [Webster 1913 Suppl.]