The Train of Thought is a train inside Riley's mind in Inside Out. It is a fantasy train with a locomotive looking like a cross between a tank engine and an electric locomotive that runs on tracks which form in front of it and disappear behind it. It goes all around Riley's mind in a more or less unpredictable way and delivers supplies like daydreams, facts and opinions, as well as memories. It is one of the only ways to get to Headquarters. The train only runs during day, however, as its conductor is on break when Riley sleeps.
After Joy and Sadness get lost in Riley's mind, they meet Bing Bong who advises them to take the Train of Thought to get back to Headquarters. From then on their journey concentrates on catching the train and then getting it to go, which turns out to be harder than they expected. However, just when they finally get onboard and on route to Headquarters, the destruction of Honesty Island destroys the train's tracks, causing it to be lost in the Memory Dump.