public int PreviousObservationsCount {get; set;}

{
  //The layer must be track aware and spatial
  if (streamLayer.TrackType != TrackType.Spatial)
  {
    // not track aware and spatial
  }
  // Note: call within QueuedTask.Run()
  {
    //get the CIM Definition
    var def = streamLayer.GetDefinition() as CIMFeatureLayer;
    //set the number of previous observations, 
    def.PreviousObservationsCount = (int)streamLayer.GetExpirationMaxCount() - 1;
    //set show previous observations and track lines to true
    def.ShowPreviousObservations = true;
    def.ShowTracks = true;
    //commit the changes
    streamLayer.SetDefinition(def);
  }
}