The Rocket Wagon is a basic red wagon with two brooms on it, one taped on each side, representing the boosters. Above the brooms are pieces are cardboard, representing the wings.
Role in the film
When Riley becomes a pre-teen, she no longer remembers Bing Bong as her childhood imaginary friend, but a manifestation of him still exists in her mindscape, and he would momentarily store his rocket away.
When Bing Bong is discovered by Joy, she instantly figures out who he is, and highlights all the amazing times Riley had with him.
After a harrowing trip through Riley's Abstract Thought, Bing Bong offers to take Joy and Sadness through Imagination Land. At the House of Cards, Bing Bong reveals that he stored away his rocket under a pile of cards, and announces his next plan with it: take Riley to the moon.
After witnessing the destruction of Hockey Island, Bing Bong offers to take Joy and Sadness to the nearest train station to board the Train of Thought, Bing Bong witnesses the Mind Workers tearing down some of Imagination Land's famous locations. He becomes even more shocked to discover that Mind Workers are confiscating his rocket. Bing Bong pleads the Mind Workers to stop, but his request falls on deaf ears, as the Mind Workers bulldoze a bunch of junk into the Memory Dump, one of which was the rocket. Because of this, Bing Bong falls into a deep melancholy. He explains to Sadness (who offers to help him) that the rocket was all that he had left of Riley, and because his rocket is gone, he feels like the Riley no longer remembers him. Luckily, thanks to Sadness consoling him, he feels better.
Later on in the film, Joy and Bing Bong fall into the Memory Dump after Family Island's slow deterioration. Needing to escape, Joy and Bing Bong remember that the rocket fell into the Memory Dump earlier, so they decided to sing the song that powers the rocket, and they successfully find it. The two sing the song as loud as they can to get out, but they fall short at every attempt. On the third and final try, Bing Bong intentionally ditches the rocket to make it lighter, and allowing Joy to escape. When the rocket slams onto the ground, the rocket breaks apart, destroying what was once left of Riley's childhood with Bing Bong, just before he fades away, forgotten, and no longer remembered.