Regarding environmental impacts of the line, this quote from the article sums it up best
“The Perris Valley Line Metrolink extension will provide an environmentally friendly commute option by utilizing an existing rail track that has been in the area for more than a century, thus avoiding significant environmental impacts,” Supervisor John Benoit said, in the news release.
For anyone who is interested, I quickly looked up the proposed train volumes. It is twelve per day according to this safety assessment performed last year:
Perris Line Safety Assessment
I live on a street in an eastern city where there are 12 buses
per hour passing my residence on weekdays.