I don't know how the builders of this sculpture did their strength calculations, but they were pretty good! For it to handle such a weight, and likely quite a good impact, it is an amazing feat of building.
Or maybe they were off by a couple orders of magnitude! Seems like a waste of material and engineering if it wasn’t supposed to support a train. But if an error a fortunate one.
Plastic, engineered properly, can be incredibly strong.
99% of plastic products are not engineered to any requirement beyond “this shape, that color”. So we’re not used to seeing proper engineeered plastic. But it’s a wonder material in skilled hands.