spriteset back_sprites {
  ... pairs for sprites for cargos and orientation ....
}

spriteset front_sprites {
  ... pairs for sprites for cargos and orientation ....
}

spritelayout concrete_platform_layout {
  ground { sprite: GROUNDSPRITE_TRACK; orientation_offset: -1; }
  building {
    sprite: back_sprites(2 * LOAD_TEMP(0));
    yextent: 4;
  }
  building {
    sprite: front_sprites(2 * LOAD_TEMP(0));
    yoffset: 12;
    yextent: 4;
  }
}