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;

}