http://www.reddit.com/r/Minecraft/comments/1n6kft/minecraft_snapshot_13w39a/ccftaslI made a circle of track 5x5. Middle of each side is activator rail. Made 4 Minecart with Command Blocks on each turn.
Two do this: setblock ~ ~ ~ redstone_block 0 replace
Two do this: setblock ~ ~ ~ air 0 replaceShoved them, they take a moment to sync up, but it's fun to watch. Note that as the entity is resting on the rail, the ~ ~ ~ relative location isn't actually the rail it is on but the block above the rail.
I just tried making a system of 3 of them. First place places activator rail, second one puts a redstone block under it, third one places golden rail. I think this screws it up though, because when they get to the starter activator rail, they dig into the ground and vanish. xD But they make rail all the way down so...
setblock ~ ~-1 ~ minecraft:activator_rail 0 replace
setblock ~ ~-2 ~ minecraft:redstone_block
setblock ~ ~-1 ~ minecraft:golden_rail 0 replaceEdit: Thinking back on the above, I should probably make the relative coordinates place them ahead. I noticed that the way these Minecarts work, they would accidentally place in front of them as well, but I suppose that won't work perfectly from below. Although making relative horizontal coordinates means this can't turn...
summon MinecartCommandBlock ~ ~1 ~-1 {Command:"setblock ~ ~-1 ~-1 minecraft:activator_rail 0 replace"}
summon MinecartCommandBlock ~2 ~1 ~-1 {Command:"setblock ~ ~-2 ~-1 minecraft:redstone_block"}
summon MinecartCommandBlock ~-2 ~1 ~-1 {Command:"setblock ~ ~-1 ~-1 minecraft:golden_rail 0 replace"}
Edit2: Got it:
Center: summon MinecartCommandBlock ~ ~ ~-3 {Command:"setblock ~ ~-1 ~-1 minecraft:activator_rail 0 replace"}
Left: summon MinecartCommandBlock ~-2 ~ ~-1 {Command:"setblock ~ ~-1 ~-1 minecraft:golden_rail 0 replace"}
Right: summon MinecartCommandBlock ~2 ~ ~-2 {Command:"setblock ~ ~-2 ~-1 minecraft:redstone_block"}
Pay attention to "f" direction value. As this only works right with relative horizontal coordinates, it can only move in one direction. You can adjust the relative coordinates to work for you, but it can't turn. Also, if it goes over holes, they screw up. Left repeater is 1 tick, right repeater is 2 tick. Probably not required, but this setup works for me.
Edit3: Hilarious side note: They seem to bounce at the end of loaded chunks. When they come back and bounce off the starting point, they start removing the leftover activator rails. This system cleans itself up, so now it'll be safe for TNT Minecarts. :D
Poll