CITY Advent: Day 21 Dec21 by Christopher Doyle on December 21, 2010 at 6:00 am Posted In: 2010 CITY Advent, Blog 21 of 24. Day Twenty One: Yellow Train Car The same set as Day Nineteen. This time in red. So go back and re-read day 19 and substitute the word “yellow” for “red”. There. All done for today.