/summon MinecartSpawner ~ ~ ~1 {
  Riding: {
    id: "MinecartSpawner",
    Riding: {
      id: "MinecartRideable",
      Riding: {
        id: "MinecartRideable",
        Motion: [0.0, 0.0, 2.0]
      }
    },
    SpawnRange: 0,
    MinSpawnDelay: 1,
    MaxSpawnDelay: 1,
    SpawnData: {
      TileID: 66,
      Time: 1,
      DropItem: 0
    },
    EntityId: "FallingSand",
    SpawnCount: 1
  },
  SpawnRange: 16,
  MinSpawnDelay: 30,
  MaxSpawnDelay: 30,
  SpawnData: {
    TileID: 137,
    Time: 1,
    DropItem: 0
  },
  EntityId: "FallingSand",
  SpawnCount: 1
}

/summon FallingSand ~ ~5 ~3 {
  TileID: 152,
  Data: 13,
  Time: 1,
  Riding: {
    id: FallingSand,
    TileID: 137,
    Time: 1,
    TileEntityData: {
      Command: "say Block B"
    },
    Riding: {
      id: FallingSand,
      TileID: 152,
      Time: 1,
      Riding: {
        id: FallingSand,
        TileID: 137,
        Time: 1,
        TileEntityData: {
          Command: "say Block A"
        }
      }
    }
  }
}