For all of you reading this wondering how you can watch it as well, I have some bad news - this movie is a short film shown only at the Ghibli Museum in Mitaka, Japan. When you visit the museum, they give you a pass to see whatever short film is playing on that day, and the films rotate every few months.
I happened to see this movie not once, but twice, the two times I visited the museum (YEARS apart). I wish I had seen another film the second time I visited, but that being said, it's a truly fantastic movie.
In the fantastical tale, Mei meets up with a baby Catbus that she can actually fit in, and the two embark on a grand adventure, meeting up with various other types of cat vehicles (including a train, an ancient plane, etc). The animation is stunning as always and based on what I remember about the music, it's delightful and appropriate.
If Ghibli ever releases these shorts to the public, I strongly suggest a watch. It's all the best elements of Totoro (the Catbus, Totoro, etc) distilled into a short film. ^_^