cool topic.
not totally sure about the premise though. is it really true that an elevated monorail of a given capacity (crush load of trainset * frequency/headway) results in a "lighter" structure than elevated light rail or even elevated heavy rail?
the structure pictured at the beginning from chicago or new york are certainly not what you'd build today. even the 1960s era structures for BART's elevated structures at the ohlone greenway are light by comparison, and while they're certainly heavier than the disney monorail, they're really an absolute worst case scenario: wide gauge, heavy rail trains in the highest seismic zone there is, built by a big conservative government agency.
placing them higher, as they are here, actually reduces their visual impact further.
at the other end of the spectrum, i'd imagine a motorless light rail train like BART's oakland airport connector is really as light as such a thing could possibly be in a seismic area.
it's not a bad structure and the uses of trusses for the main spans is something i haven't seen in a long time.
aren't there problems with monorails from friction, lateral loading, turn radius etc?