switch(FEAT_TRAINS, SELF, switch_vehicle_part, [
  STORE_TEMP((getbits(extra_callback_info1, 8, 8) < NNN ? CB_FLAG_MORE_SPRITES : 0) | PALETTE_USE_DEFAULT, 0x100),
  getbits(extra_callback_info1, 8, 8)
]) {
  0: vehicle_part0;
  1: vehicle_part1;
...
  NNN: switch_part_NNN;
}