The mirror acts as the conduit for Tulip and M.T. to physically swap places, allowing M.T. to enter the "Prime" world and Tulip to enter the "Mirror World".
Later, it is used as a tool to help M.T. evade the Reflection Police (Mace and Sieve), who hunt reflections that try to live independently from their humans. Episode Impact
In the seventh episode of Infinity Train Season 1, titled the "helpful piece" is a small, reflective handheld mirror that Tulip uses to interact with her reflection. Role of the Helpful Piece [S1E7] The Chrome Car
It introduces the concept of the Mirror World and reveals that One-One’s reflection, "Mirror One," speaks with a singular, combined voice, hinting at his mysterious past. Infinity Train S1E7 "The Chrome Car" Recap - TV Tropes
After Tulip shows empathy and helps M.T. gain her freedom, her glowing green hand number drops significantly (to 3), signaling her personal growth. The mirror acts as the conduit for Tulip and M
M.T. becomes the main protagonist of Infinity Train Book 2.
This episode is a major turning point for the series, as it: Later, it is used as a tool to help M
It allows Tulip to see and speak directly with her reflection, who eventually reveals herself as a sentient being named Mirror Tulip (or M.T. ).