Grid one_grid = doc.GetElement(new ElementId(20637)) as Grid;
var start = one_grid.Curve.GetEndPoint(0);
var end = one_grid.Curve.GetEndPoint(1);
Transaction t = new Transaction(doc, "create_model_curve");
t.Start();
Line line = Line.CreateBound(start , end );
Plane plane = Plane.CreateByOriginAndBasis(start, XYZ.BasisX, XYZ.BasisY);
Autodesk.Revit.DB.SketchPlane skp = Autodesk.Revit.DB.SketchPlane.Create(doc, plane);
Autodesk.Revit.DB.ModelCurve mline = doc.Create.NewModelCurve(line, skp);
t.Commit();