Placing blocks on minecarts

It would be awesome if you could place blocks ON minecarts. I have had lots of great ideas about creating cars and trains that actually move, but it's to hard to create them using only Redstone mechanics.

Please add this feature so that we can make cooler ways of transportation (In my opinion, the coolest method of transportation is to put a horse or a pig in a minecart and ride it). Because in Minecraft, I would like a cool way of transportation in my city instead of just a boring old minecart.

:)