If a non-railed Minecart goes over or tries to land on a bottom slab and there's a rail below that bottom slab, the Minecart will get sucked through the bottom slab and onto the rail below.

Not the Minecart Item but the Minecart entity that the player can ride, i've tested this with Hopper Minecarts too and the same thing happens.

As a player you'll get suffocated because your head is inside the bottom slab when you get sucked through the block.