One, or most commonly known as One-One, is one of the main characters of the Cartoon Network series Infinity Train. He is the deuteragonist of Book One - The Perennial Child, a supporting character of Book Two - Cracked Reflection, the unseen overarching character of Book Three - Cult of the Conductor, and a major character of Book Four - Duet. He is also the main protagonist of The Train Documentaries.
He is the robotic companion of Tulip Olsen who is trying to help her find a way back home. One-One can split into two halves, each with its own personality. The two halves are called Sad-One and Glad-One by the production staff, but as a single unit, they are merged into One, the original identify of One-One. One-One is also the true conductor of the Infinity Train and regains his position at the end of Book One.