They specifically mention railway noise. I wonder whether there's something special about railways, or whether it would work for other noise as well? Especially roads with cars on them?
At least the linked youtube video in this comment [1] does not mention anything about frequencies.
[1] https://news.ycombinator.com/item?id=40384778
EDIT: I was wrong about the video. There is _no mention_ about frequencies.
Highways care a lot less about location. Place them a one-minute car ride outside the city, and its noise becomes basically irrelevant. The city grows and swallows the highway? Just place an industrial area or mall between the highway and any homes. When homes aren't an arms-length away you can get away with far more primitive noise reduction.