Yes, you read that title. Minecarts can now be edited with a third party program to show any block inside of it (it will not take on the characteristics of this block), as well as make it take on the characteristics of any cart, and this can be done in Creative Mode.
How does it work?
You can shift right-click on a Minecart in Creative Mode with a certain block (except for chests, furnaces, hoppers, TNT), to place it inside a minecart. This allows you to create new ideas for adventure maps, and as a way to place Monster Spawners inside Minecarts. You cannot place items, cobwebs, technical blocks (having an end portal inside a minecart never makes any sense!), flowers (including poppies and most from 1.7 snapshots), Vines, leaf blocks, crops, and other vegetation (exception is for logs).
Plus, you cannot place repeaters and comparators inside a minecart. Same goes for doors, pressure plates, buttons, and levers, and torches and redstone torches, and rails of any type.
You can also place mob heads inside minecarts. You can also move light-producing blocks (excluding torches), but that can be hard to code, and could sometimes cause lag. OH NO.
Building blocks in minecarts are often used for portable stepping stones when used in adventure maps. Redstone blocks in minecarts can be used as a portable activator when it hits adjacent redstone and activator rails.
Can you use them in survival mode?
Yes, probably, except for TNT, Storage, Powered, and Hopper minecarts - which have to be made through crafting.
This can be a great idea for creative mode, if not, constructive criticism is appreciated. Support? Give it a rep, and post your own ideas!
I like this idea, but why not have it in survival? I don't think it would be gamebreaking or anything. Also, why not be able to place a chest/hopper/furnace/TNT inside to make it into the respective cart? I think it would be a nifty addition aswell. It also might be nice to allow a hopper below a cart to take the block out of it, and a hopper above to move a block into a cart? Just some thoughts. :3
Although it is possible to spawn minecarts in the most recent snapshots with commands and a little knowledge of data values, I support the other ideas like portable light sources or redstone block minecarts as they are not in the game yet and would be very useful for map makers.
"Colder than the snow that blankets the earth" "Colder than the ice that cover the stones and freezes the rivers" "Colder than the north wind blowing through a winter-bound forest" "Colder than all these is the smile of a Shade of the Dark"