At some point before the events of the third season, Belle arrived on Sodor to help put out fires and joined the Sodor Search and Rescue Team.
Belle helped to put out the fire at the warehouse complex in Cronk after The Assassin detonated a bomb.
Belle is a big helpful steam engine that can shoot water from her boiler-mounted cannons to help put out fires.
• The Ones Who Know Part 3 (does not speak)
- Belle has yet to speak. (However she did speak in the original cut of A Tale of Two Trains)