Minecraft wiki entry for Development_versions, associated text: Can only be obtained using /give @p 422, /give @p hopper_command_block or /summon MinecartCommandBlock.
Activated by activator rails.
Run their command every so often as long as they are powered - Running a command block minecart across a single powered activator rail at full speed will run its command thrice.
Can be success-tested while on a detector rail with a redstone comparator next to the rail (just like how the minecart with chest and minecart with hopper can be fill-checked).