Story is about Mei and her friend the kittenbus (the child of the original catbus from My Neighbor Totoro).
Mei is just small enough to ride in the kittenbus, which is only big enough to stir up dust devils, rather than making whole fields of rice sway in its breeze. One night they have an adventure in which they fly into the forest with many other cat-based vehicles, including many buses and trains. There they meet Totoro and many similar spirits, all heading for a gigantic catliner. This liner cruises off into the sky and the kittenbus takes Mei home.

Source: nausicaa.net/miyazaki/museum/mei

Note: The third in a series of short films produced to screen exclusively at the Studio Ghibli Museum in Japan.
There has been no commercial release for this film, which premiered at the museum on 1 October 2001.

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Same setting: Tonari no Totoro
Other: Kujira Tori
Other: Koro no Dai Sanpo