Minecarts can pass through non-full height blocks

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.