The Block Changer sign can change the types of blocks displayed in a minecart. For example, a storage minecart shows a chest - here the displayed block type is a chest. You can change the type of block from a chest to something else using this sign. All minecart types support this sign, and the displayed block types are persistent through server restarts.
First line: [train] to work on the entire train, [cart] to work on a single cart
Second line: blockchanger
Third and Fourth lines: The block types to set (see: Item format)
The types set repeat: in the above example it repeats every 5 minecarts. The first two minecarts get a wooden block, the next three minecarts a stone block. Not specifying an amount is seen as infinite - which means that the specified block type counts for all carts. The following example shows a train set to use a diamond block for the first cart, a gold block for the next two carts, and empty (air) for all the remaining carts:
If you need more advanced settings, you can combine a [cart] blockchanger with a Switcher to individually set the block type based on per-cart properties. If you need to specify a lot of block types, you can use Item Shortcuts to define a lot of block types at once.