One-One is the deuteragonist of Book One and a supporting character of Book Two in the Cartoon Network series, Infinity Train. 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 called One-One. One-One is also the true conductor of the Infinity Train and regains his position at the end of Book One.
One-One is a white, spherical robot with a black stripe that runs down the entire circumference of its body. On the front, the black stripe has two white dots that appear to be Sad-One and Glad-One's eyes when they are fused. One-One has four retractable spider-like legs.
The body can easily separate into two identical and independent parts and can reattach itself together again. Each half is semi-spherical in shape with a black line at their base with a single white dot, their eye. The flat base of each half has four spider-like legs, similar in structure to when it was whole.
- One-One is the most recurring character in the series.