The Collaborative International Dictionary of English v.0.48:

Trail \Trail\ (tr[=a]l), v. t. [imp. & p. p. Trailed; p. pr. &
   vb. n. Trailing.] [OE. trailen, OF. trailler to trail a
   deer, or hunt him upon a cold scent, also, to hunt or pursue
   him with a limehound, F. trailler to trail a fishing line;
   probably from a derivative of L. trahere to draw; cf. L.
   traha a drag, sledge, tragula a kind of drag net, a small
   sledge, Sp. trailla a leash, an instrument for leveling the
   ground, D. treilen to draw with a rope, to tow, treil a rope
   for drawing a boat. See Trace, v. t.]
   [1913 Webster]
   1.
      (a) To hunt by the track; to track.
      (b) to follow behind.
      (c) To pursue. --Halliwell.
          [1913 Webster +PJC]

   2. To draw or drag, as along the ground.
      [1913 Webster]

            And hung his head, and trailed his legs along.
                                                  --Dryden.
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            They shall not trail me through their streets
            Like a wild beast.                    --Milton.
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            Long behind he trails his pompous robe. --Pope.
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   3. (Mil.) To carry, as a firearm, with the breech near the
      ground and the upper part inclined forward, the piece
      being held by the right hand near the middle.
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   4. To tread down, as grass, by walking through it; to lay
      flat. --Longfellow.
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   5. To take advantage of the ignorance of; to impose upon.
      [Prov. Eng.]
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            I presently perceived she was (what is vernacularly
            termed) trailing Mrs. Dent; that is, playing on her
            ignorance.                            --C. Bronte.
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The Collaborative International Dictionary of English v.0.48:

Trailing \Trail"ing\,
   a. & vb. n. from Trail.
   [1913 Webster]

   Trailing arbutus. (Bot.) See under Arbutus.

   Trailing spring, a spring fixed in the axle box of the
      trailing wheels of a locomotive engine, and so placed as
      to assist in deadening any shock which may occur. --Weale.

   Trailing wheel, a hind wheel of a locomotive when it is not
      a driving wheel; also, one of the hind wheels of a
      carriage.
      [1913 Webster]

WordNet (r) 3.0 (2006):

trailing
    n 1: the pursuit (of a person or animal) by following tracks or
         marks they left behind [syn: trailing, tracking]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "trailing":
   bugging, chase, chasing, cloak-and-dagger work, consecutive,
   counterespionage, counterintelligence, dogging,
   electronic surveillance, espial, espionage, follow, follow-up,
   following, heeling, hounding, hue and cry, hunting, intelligence,
   intelligence work, military intelligence, observation, prosecution,
   pursual, pursuance, pursuing, pursuit, quest, searching,
   secret police, secret service, seeking, sequel, sequence, series,
   shadowing, spying, stakeout, stalking, succeeding, surveillance,
   tailing, tracking, tracking down, wiretap, wiretapping