if you drop any minecart (tested with TNT and regular) on a slope and give it some momentum with powered rails it will sink into the floor at the end of the track and turn black.
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duplicates
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#1 And #2 are glitched minecarts.
#3 minecart railway: situation with slopes
#4 minecart stuck in first "hill"
#5 minecart stuck in sand/ground/floor/any-type-of-floor-at-the-end-of-the-rail

Duplicate of MC-8004