If you place a Minecart directly on top of a Slime block, the shadow of the minecart will cause the top texture of the slime to be transparent and show what's behind it without the slime texture obscuring it.  See the screenshots attached.

You can see this occurring in this video here as well at 8:10: https://www.twitch.tv/videos/495712598?t=00h08m10s