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“I just love crushing up rocks!"
"Just don't spill any on the tracks, Ballast Brewster. I don't want to be slowed down by anything today!"
"I won't...”
―Brewster and Koko

Runaway Koko is the fourth episode of the fourth season.


Brewster tells Koko and Wilson that he is making ballast with Speedy. Koko then goes to practice with Hanzo, where she practices letting go of her carriages in emergencies. Wilson is sent by Jackman to practice with Calley shooting nets out off Chug Patrol 7. Koko is doing her rounds. She arrives at Covered Bridge when her boosters go off. She decides to release her carriages at the next platform. Her boosters go off again so she gets Harrison to take over. Jackman alerts Chug Patrol. Hanzo comes to rescue Koko. He tries connecting a track wagon, but it fails. She then heads to the mountains to slow down up hill. Meanwhile, Wilson has prepared some nets using Chug Patrol 7, but that fails. Koko then heads to Rocky Ridge Quarry and tells Brewster to drop some ballast on the track. The Ballast slows her down and the dust clogs her engine. Hanzo and Calley couple up to her. Hanzo notices that she stayed calm throughout.


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  • In the UK and US cerdits, Sacha Dhawan and Trevor White go uncredited for voicing Eddie.
  • Koko's horn is missing when Brewster arrives at the Working Wheels Yard.
  • Calley's third window is missing once Koko arrives at the quarry.