So engineers turned to steam engines to return the carts back up the line to the slate quarries before they were returned down the line under their own gravity.
This exploits one of the predictions of Einstein's general theory of relativity: that the path of a beam of light (which is a straight line in empty space) is bent inwards by the gravity of a massive object.
If an object floating through space passes near the line of sight between the earth and a distant star, its gravity should, according to the theory of relativity, bend and focus the light from that star.