Steam, also called Stim, is a brown cat with light brown fur and striking purple eyes. She dons a light blue striped cap, a matching long-sleeved shirt with a red tie, and sturdy blue dress overalls. As the engineer and mechanic of the locomotive Cactus, she tirelessly works to complete a tunnel beneath the Rookery Mountains, aiming to link Cube Town and Cubic City. Her passion for the project fuels her sharp reactions to disruptions, often sparking frustration, though she consistently forgives offenders once issues settle.

Three rivals—Pecola, Chewy, and Rudy—vie for her attention, sparking recurring friction. Despite this, Steam remains approachable, fostering camaraderie with peers like Cori, whose professional collaboration mirrors her own work ethic. She balances her disciplined focus with a playful side, joining soccer matches with friends, undeterred by age.

When challenges to her expertise arise—such as a derisive remark about her progress—her irritation flares, yet resolution follows swiftly, highlighting her pragmatic nature. Beyond the rails, she contributes to community events and aids neighbors, though her primary drive stays fixed on perfecting Cactus and realizing the tunnel’s promise.