Most of the 2 speed axles have two ratios- - - -"Slow"- - - -and "Really slow!" Chances are that the coach builder that made the fire body swapped in a 2 speed to get the rig moving when it had a tank full of water onboard.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!