BC7004
The BC7004 spawns a cart when powered.
This sign can be placed under a track. To spawn a cart, the block on the right or on the left of the sign must be powered with some redstone dust on it. It can be done with a button.
The 4th line of the sign may contain a keyword to tell the type of cart to spawn: normal, storage cart, hopper cart, furnace cart.
Line #1 : free
Line #2 : [BC7004]
Line #3 : free
Line #4 : [storage, hopper, furnace] or empty for normal cart
BC7004 was added in release 2.3.1