[Duplicate of MC-194363]

When you place a minecart going down a rail it rolls down. If one block above and ahead (in the direction the cart moves) there is a block, the cart bumps into it. So far so good. As soon as you remove the block the cart moves. However, if you have an upside-down slab instead of a block, even though the top of the cart is around .25, and the bottom of the slab is at .5,  the slab stops the cart. I have tested this bug and it appears in 1.8.9, 1.15.2, and 1.16.1, meaning this is probably old.