A rail jet was a hovertrain that traveled on one rail and transported ammunition, supplies and sometimes troops.
Rail jets first appeared in "Destroy Malevolence," the fourth episode of the first season of the Star Wars: The Clone Wars television series, which aired on October 17, 2008.
The design of the rail jet was inspired by the Trade Federation's Multi-Troop Transport seen in the 1999 film Star Wars: Episode I The Phantom Menace.