Duncan is a yellow narrow guage engine.
- Season 1 - Luke and the Special Party and Making Fun
- Season 2 - Don't Be Late!, Duncan's Predicament, Welcome Smudger, and Sir Handel and the Ghost Engine
- Season 3 - Peter Sam and the Snow (cameo), Duncan and Luke, The Truth is Revealed, An Earned Apology, and Duke's Final Run (cameo)
- Season 4 - Trapped
- Season 5 - Down the Bridge
- Season 6 - Sherwood the Infamous (cameo),