Here is the grade-separated branch that allows aerial networks. No other system in the world, existing or proposed, is anything like it. Transit experts wouldn't believe until it was demonstrated (see video). Here a train is approaching the merge switch from a two-way branch line. In the foreground, another train has passed through the diverge switch and continues along the main line. Monorails can't turn nearly this sharp, so they just don't fit above downtown streets.