Koko is a Japanese electric engine. She is an advanced Trainee and is one of the main characters.
Unlike Wilson and Brewster, Koko is completely electric and uses her own power directly from the tracks beneath her. She'll often challenge others to a race, knowing full well that they don't stand much of a chance against her. Koko loves going flat out, whooshing ahead of the others, and has yet to learn that speed isn't everything. Koko is very confident and loves to explore and have adventures. Although mischievous, she is well meaning and gets upset if she causes her friends to get into trouble. Koko will always own up or apologize and try to put things right, once she has understood the full implications of her actions. She is an express engine, built for speed and pulling passengers.