public double RailThickness {get; set;}

{
  // must be on MCT 

  var railFilter = new LasPointRailSelectionFilter();
  railFilter.VisiblePoints = true;
  railFilter.ClassCodes = new List<int>();  // empty list means all classification codes
  railFilter.FilterGeometry = polygon;

  // configure a few of the properties and accept defaults for others
  railFilter.SearchRadius = 0.5; // meters
  railFilter.RailThickness = 0.2; // meters
  railFilter.MaximumLength = 225; // meters

  var railSelCount = await lasDatasetLayer.SelectAsync(railFilter, SelectionCombinationMethod.New);
}