Minecraft – How to place redstone using Command Blocks

There is this one character (I don't know what it is yet, either), that sometimes, somehow, makes its way into commands. Try going to the beginning of your /testfor command, and holding -> until you reach the end. There may be a spot where your cursor stops moving, and there should be a zero-width invisible character there. It shows up in regular editing software, I always check if my commands aren't working.

You must add the id:FallingSand tag to each block you want to stack. So, you're command should be:

/summon FallingSand ~ ~1 ~ {Block:redstone_block,Time:100,Riding:{Block:command_block,Time:100,TileEntityData:{Command:/say test},id:FallingSand}}

Explanation

It is like when making any entity riding another entity. When not adding the id tag to the stacked entities (FallingSand blocks), the command block will not know which entity it should summon that the FallingSand is riding on. Like for example, when you want to make a Zombie riding a Bat and do /summon Zombie ~ ~ ~ {Riding:{}}, it will not know that you want the entity that the Zombie is riding must be a Bat, but when you do /summon Zombie ~ ~ ~ {Riding:{id:Bat}}, it will know that the Zombie should ride a Bat. The FallingSand is used in the same way as any other entities when riding each other.