The Collaborative International Dictionary of English v.0.48:

Trunk \Trunk\, n. [F. tronc, L. truncus, fr. truncus maimed,
   mutilated; perhaps akin to torquere to twist wrench, and E.
   torture. Trunk in the sense of proboscis is fr. F. trompe
   (the same word as trompe a trumpet), but has been confused in
   English with trunk the stem of a tree (see Trump a
   trumpet). Cf. Truncate.]
   1. The stem, or body, of a tree, apart from its limbs and
      roots; the main stem, without the branches; stock; stalk.
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            About the mossy trunk I wound me soon,
            For, high from ground, the branches would require
            Thy utmost reach.                     --Milton.
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   2. The body of an animal, apart from the head and limbs.
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   3. The main body of anything; as, the trunk of a vein or of
      an artery, as distinct from the branches.
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   4. (Arch) That part of a pilaster which is between the base
      and the capital, corresponding to the shaft of a column.
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   5. (Zool.) That segment of the body of an insect which is
      between the head and abdomen, and bears the wings and
      legs; the thorax; the truncus.
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   6. (Zool.)
      (a) The proboscis of an elephant.
      (b) The proboscis of an insect.
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   7. A long tube through which pellets of clay, p?as, etc., are
      driven by the force of the breath.
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            He shot sugarplums them out of a trunk. --Howell.
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   8. A box or chest usually covered with leather, metal, or
      cloth, or sometimes made of leather, hide, or metal, for
      containing clothes or other goods; especially, one used to
      convey the effects of a traveler.
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            Locked up in chests and trunks.       --Shak.
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   9. (Mining) A flume or sluice in which ores are separated
      from the slimes in which they are contained.
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   10. (Steam Engine) A large pipe forming the piston rod of a
       steam engine, of sufficient diameter to allow one end of
       the connecting rod to be attached to the crank, and the
       other end to pass within the pipe directly to the piston,
       thus making the engine more compact.
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   11. A long, large box, pipe, or conductor, made of plank or
       metal plates, for various uses, as for conveying air to a
       mine or to a furnace, water to a mill, grain to an
       elevator, etc.
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   Trunk engine, a marine engine, the piston rod of which is a
      trunk. See Trunk, 10.

   Trunk hose, large breeches formerly worn, reaching to the
      knees.

   Trunk line, the main line of a railway, canal, or route of
      conveyance.

   Trunk turtle (Zool.), the leatherback.
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WordNet (r) 3.0 (2006):

trunk line
    n 1: line that is the main route on a railway [syn: trunk
         line, trunk route]
    2: a telephone line connecting two exchanges directly

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "trunk line":
   L, WATS, WATS line, branch, cable, cable railway, co-ax,
   coaxial cable, cog railway, concentric cable, el, electric railway,
   elevated, elevated railway, embankment, feeder, feeder line,
   gravity-operated railway, horse railway, junction, light railroad,
   line, main line, metro, monorail, party line, private line,
   rack railway, rack-and-pinion railway, rail, rail line, railroad,
   railway, roadbed, roadway, sidetrack, siding, street railway,
   streetcar line, subway, switchback, telegraph cable,
   telegraph line, telephone line, terminal, terminus, track, tram,
   tramline, trestle, trolley line, trunk, tube, turnout, underground,
   wire line