Switchboard

Switchboards are a kind of platform in Super Mario 3D Land. They're green and have two arrows, pointing left and right, each one on a pad. If Mario or Luigi steps on one of this pads, the arrow he stands on will turn yellow, and the Switchboard will move over rails to the indicated direction.

Switchboards first appear in World 1-3, and appear in large numbers in World 1-4, Special 3-1 and Special 5-3. If the platform touches with a small block at the end of the trail, it will simply bump and stop. A trail ending without a block will cause the Switchboard to fall down.

Trivia
A similarly functioning type of platform with four arrows and directions that it can move in appears in Hazy Maze Cave in Super Mario 64 and Super Mario 64 DS.