public sealed class RangeTrack : Track 

public void CreateRangeKeyframe(ArcGIS.Desktop.Mapping.Range range, TimeSpan atTime)
{
  var mapView = MapView.Active;
  if (mapView != null)
    return;

  var animation = mapView.Map.Animation;
  var rangeTrack = animation.Tracks.OfType<RangeTrack>().First(); //There will always be only 1 RangeTrack in the animation.
  rangeTrack.CreateKeyframe(range, atTime, ArcGIS.Core.CIM.AnimationTransition.Linear);
}