"Mixed Signals" is the sixth episode of the first season.
Thomas loves his branch line when he travels the big harbor, where Thomas' little tank engine sister, Percy is talking to her tank engine brother, Benny. She tells Thomas about mixed signals. Benny has a brilliant idea to shunt freight cars in the station yard. The next day Thomas and Percy need drink, but the water tower is out of order, so they their crew decide to fill china clay. Percy enjoys it, but soon gets a pain in her boiler. She puffs in a siding feeling as if her boiler is about to burst. The conductor telephones for an engine inspector, and surrounds Thomas and Percy with warning signs that say "Danger, Keep Away". The Inspector and the Fat Controller soon find the cause of the problem - Thomas and Percy's drivers and firemen have unloaded china clay with the water. Benny came up with china clay cars and sees mixed signals. He backs up and tells Thomas and Percy about them. They decide that they aren't keen with china clay.