Many public transport projects are dependant on the community around allowing them to be built, mitigating the impact of said projects allows more public transport to be built.
E.g. a tram doesn't go very fast and is also pretty quiet. A train usually runs a lot faster, and causes more noise pollution.
If you can mitigate the noise, then you'll probably be able to build the railway instead of the tram. Allowing more capacity, higher speeds, and a better public transport solution.