The Keyboard Car is a car that appears in the episode "The Corgi Car." It is a car that appears to be in outer space with floating musical instruments and a piano pathway that leads from door to door which Tulip slides along.
This room was actually shown in the puzzle on Cartoon Network's website.[1] Dropping on June 18, 2019, Cartoon Network posted a link on their social media accounts to a website displaying this room with the piano labeled and interactable.[2] Upon playing the Infinity Train theme with cords D, B, G, & F#, the trailer played and gave us a first look at the series.
Screenshot and puzzle key to unlocking trailer on Cartoon Network website [1]