It's a Steam Tank!
(Rule)Description: For all intents and purposes the Rat Tank acts and uses the same rules as the famed Empire Steam Tank, which can be found in the Warhammer Annual 2002 (UPDATED IN CHRONICLES 2004, P.48). The Rat Tank has 25 Hull Points.
The rules are massive, so just check the book ("Warhammer Chronicles 2004" by Alessio Cavatore, p.48-53) or follow the link https://6th.whfb.app/special-rules/steam-tank