*man, I love retro-futuristic art
The challenge here is to wank the prevalence of Monorail systems in the US from 1950-onward as much as possible.
Bonus points if you can devise a way for it become the second most popular mode of transportation.
Here's a pretty good starting point
If New York didn't scrap the proposed 1930s monorail in favor of the elevated train system it might have jumped started other metropolitan area adopting the same.
LA accepting Walt Disney's offer would also be a good starting point.
Practically none, in either case. There are a handful of more or less successful demonstration systems in a few cities (Tokyo, for example), but they're largely tourist attractions. The largest monorail system I am aware of is Chongqing's, which actually has somewhere like 100+ miles of monorail trackage. The trouble is that it's a special case, apparently because of the geography of combining a deep and wide river valley with tall mountains nearby, so it's hard to see how to replicate it.I'm writing this from a state of pure ignorance about the advantages/ disadvantages of using one rail as opposed to two for trains. There are also relevant things that I don't know, such as how many monorail systems were actually built in the US IOTL, and how many were built outside of the US.
*monorail intensifies*Now I have that stuck in my head.....