Unlike the well - known duo - rail system , there are many rail - guided transport options which have been described as monorails .
Tracing the history presents a demarcation problem regarding what should be included and what should be omitted .
Unlike the well - known duo - rail system , there are many rail - guided transport options which have been described as monorails , so that tracing the history presents a demarcation problem regarding what should be included and what should be omitted .