All types of minecarts can pass through a block if it's not a full-block, as long as there's any rail under that block.
This happens when a minecart falls on the block, or a rail is on that block and a minecart is on the rail, then you break the rail, or a minecart is on that block, and you place any rail under the block.
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I don't get it. Please provide screenshots.

If you break the rail that the cursor is pointing at, the minecart won't stay on the soul sand(which it should), it'll "teleport" to the rail below the soul sand instead. It happens on all types of minecarts, rails, and the blocks that without a full-block hight.