Short answers, pulled from the story.
Ai and Yu Hayakawa are twelve-year-old twins spawned from Chaos itself in the aftermath of a cataclysm that erased the worlds of Kaze and Makenshi. They are living fragments of the ultimate destructive force Omega and the source of the power that Earl Tyrant sought to consume.
The Ghost Train is a vehicle that did not run on rails and served as the primary mode of transport for the first half of the series. It carried the protagonists through the surreal landscapes of Wonderland while being pursued by fragments of the destructive being Omega.
The planned fifty-two episode run of the series was cut to twenty-five episodes due to a combination of low ratings and the financial failure of Final Fantasy: The Spirits Within. This decision left the story unfinished and the continuation of the story has been released in a variety of other media including printed and web novel series, manga, radio dramas, and video games.
The series aired on TV Tokyo's Network 6 on weekly on Tuesday 6:30PM starting on the 2nd of October 2001. A promotional DVD titled Final Fantasy: Unlimited Prologue Phase.0 was released on the 30th of January 2002.
The music of Final Fantasy: Unlimited was composed by Nobuo Uematsu, Shirō Hamaguchi, and Akifumi Tada. All music and arrangements were made by Shiro Hamaguchi unless where otherwise specified.