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.]

WordNet (r) 3.0 (2006):

sidetrack
    n 1: a short stretch of railroad track used to store rolling
         stock or enable trains on the same line to pass [syn:
         siding, railroad siding, turnout, sidetrack]
    v 1: wander from a direct or straight course [syn: sidetrack,
         depart, digress, straggle]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "sidetrack":
   L, avert, bear off, branch, bypath, byway, cable railway,
   cog railway, deflect, departure, deviation, digression, discursion,
   distract, divert, draw aside, draw off, ease off, edge off, el,
   electric railway, elevated, elevated railway, embankment, episode,
   excursion, excursus, feeder, feeder line, fly off, gee, glance,
   glance off, go off, gravity-operated railway, haw, head off,
   horse railway, jib, junction, light railroad, line, main line,
   make way for, metro, monorail, move aside, rack railway,
   rack-and-pinion railway, rail, rail line, railroad, railway,
   roadbed, roadway, sheer off, shove aside, shunt, shy, shy off,
   side, side path, side road, sidestep, siding, sidle,
   steer clear of, step aside, street railway, streetcar line, subway,
   switch, switchback, terminal, terminus, track, tram, tramline,
   trestle, trolley line, trunk, trunk line, tube, turn aside,
   turn away, turn back, turnout, underground, veer off