Monorails are a transport systems. While other transport systems are well catered to by model manufacturers, monorail systems are not. A number of models of monorail systems have been manufactured, and some are still. Generally they fall into the category of a 'toy' rather than a scale model.
Construction toys such as Lego and Meccano have regularly featured monorails among the models to be made by these systems. At one time Lego even had monorail system packs.
A long running system in the German town of Wuppertal is supported by some very high quality models.
References
Monorail modelling Wikipedia(Text) CC BY-SA