Stacked minecarts have variable visual behavior based on the presence of a rail

The bug

Stacked minecarts have variable visual behavior based on the presence of an underneath rail.

Demonstration

2016-06-21_10.15.22.png shows carts stacked with a rail underneath appear as just one cart.
2016-06-21_10.15.58.png shows carts stacked without a rail underneath will actually appear to stack.