class_docstr: Merges several TrackVis track files into a single track
  file.
  
  An id type property tag is added to each track in the
  newly merged file, with each unique id representing where
  the track was originally from. When the merged file is
  loaded in TrackVis, a property filter will show up in
  Track Property panel. Users can adjust that to distinguish
  and sub-group tracks by its id (origin).
  
  Example
  -------
  
  >>> import nipype.interfaces.diffusion_toolkit as dtk
  >>> mrg = dtk.TrackMerge()
  >>> mrg.inputs.track_files = ['track1.trk','track2.trk']
  >>> mrg.run()                                 # doctest: +SKIP
code: def _list_outputs(self):
      outputs = self.output_spec().get()
      outputs["track_file"] = os.path.abspath(self.inputs.output_file)
      return outputs
  
file_path: nipype/nipype/interfaces/diffusion_toolkit/postproc.py
imports: import os
  from ..base import TraitedSpec, File, traits, CommandLine, InputMultiPath, CommandLineInputSpec
label_desc: Use image node_id 1 for calling the TrackMerge obj's underlying member method code with example usage: obj._list_outputs() and returns: outputs