>>1008215you forgot
>entirely elevatedas long as your building at grade or underground monorail adds nothing to what a regular rail-based subway would do, but instead switches are complex and expensive (you need switches every couple of stations to allow partial services in case of a breakdown, especially if your lines get longer; also there's switches to allow trains go from one line to another to access depots, since large subways have usually a few centralized depots and service branch lines between lines).
Also you'll likely be using proprietary technology, which adds expense that a standard off-the-shelf system does not. I'd also bet that maintenance is more expensive for monorails than regular rail, except, again, in the case of an elevated line, in which case the monorail is basically part of the structure, while a regular rail line sits on top of a structure.
That other anon hit the nail on the head. Monorails are ok if your system is:
>simple (no branches)>self contained>entirely or mostly elevated 