Probably that as you scale up the size linearly, the volume, and thus weight goes up as the cube, but the surface area goes up as the square.
Since this design relies on closing the circuit by having the magnets rub the wire, as you scale up you have an area in contact with the wire (surface area) going up as X^2, but the downforce due to gravity going up as X^3.
You could add wheels and metallic contact brushes to scale it up. Keep in mind copper is really expensive, and you'd be scaling that up nontrivially too, or the current would fry the wire (I'd think the mass of the copper would go up as the cube too, at least to accelerate at the same rate--acceleration depends on the mass since F=MA, and the mass of the train went up as the cube).
Since this design relies on closing the circuit by having the magnets rub the wire, as you scale up you have an area in contact with the wire (surface area) going up as X^2, but the downforce due to gravity going up as X^3.
You could add wheels and metallic contact brushes to scale it up. Keep in mind copper is really expensive, and you'd be scaling that up nontrivially too, or the current would fry the wire (I'd think the mass of the copper would go up as the cube too, at least to accelerate at the same rate--acceleration depends on the mass since F=MA, and the mass of the train went up as the cube).