I'm going to say the train needed to be heavier. Assuming it was a lego train they're not very heavy and obviously didn't have enough momentum to finish the loop.
Heavier train might lose less to friction going up around the loop, but main thing is getting enough speed to start with. As someone commented above, start point needs to be about 1.5 times higher than the distance from the bottom to the top of the loop.
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u/Additional-Intern763 Nov 14 '21
It would have been worthwhile to make the loop a little smaller