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Not sure how gravity work, but I can imagine there will be slight variances in gravity, for place to place? I would expect this on either side of a tall, straight cliff.
In vacume, a perfectly inelastic ball rolls back and forth in a dito "halfpipe", over the deep nothing, and back. Actually, that wouldnot work. The ball would need to roll up on the low gravity side. and come down on the heavy side. The 2 flat sections of the flat loop rollercoaster would be to transition between low and high gravity, where velocity is maintained. Less velocity lost above the ravine, that make any theoretical sence?
Difference from all the permanent magnet devices in the world would be the "straight down" nature of gravity vs. the out-and-in of the magnet.

And man-made: the deepest we can drill straight down, topped off woth lead.  Would we be able to create a greater local gravity than found in nature? A bit further down, we'd keep a larger drill hole filled with nothing, to get twice the difference. The we place the little rollercoaster in a vacume from the one hole to the other.
How sentive is gravity to local density-to-core figures?

And, what I just wondered, what is gravity in the absolute core of earth, imagine we'd reach it, and build a gas bubble there? Weightless, or would there be a gravity vacume, which would have a structure explode with the force of one g?

Thanks for your thought. It's good to understand gravity when trying to fool it.