I know highway and aerial highway, the above-surface one. However, I do not know how to say aerial "subway" in the US. Thank you very much.
Chicago has "The El" (which is an elevated train).
Seattle has "The Monorail" (which is an elevated train that rides on a single, wide, concrete rail.)
Many cities have elevated portions of their light rail systems. Although the underground portions are often called subways, and the ground-level portions are sometimes called streetcars or trolleys, most places do not have different names for the ground-level and elevated portions of the system. For example:
- Seattle has "Link light rail" that has all three kinds of track.
- The San Francisco Bay Area has "BART", which also has all three kinds of track.
XKCD has a collection of subway maps; many of the route networks include elevated sections. Most of the "tunnels", "ferries", and "submarines" on the map are as fictional as teleporters and wormholes.
In general, I would call that an elevated train or elevated railway, although as Jasper mentions, many places have local names for their own particular elevated trains.
@stangdon, thank you very much. My comment is too long because of links, so I write it in the answer.
In fact, I found the phrase on a CNN news.
Your answer causes another problem. For years I have learned English from CNN news, which should be authoritative and correct enough to learn from, I believed. Now I realize that I may learn weird English from them. Would you please check the accuracy of the phrase aerial highway in the situation?
The highway is not elevated at some portion to cross bridges or other roads. All of it is literally built above ground to save land usage in Taipei, one of the most crowded cities on earth, as shown in figure. You can see that there are three layers of road. Two of them are elevated and you can see them on the picture. One of them is block by the embank wall and you can not see it, but it indeed in there.
Or, a big or, is the reporter just try to do a authoritative tone by adding rare words?