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 = geometry;

// 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);