Thanks for the information crzwdjk. At the community board meeting I attended where they presented this, MTA predicted an immediate 50% capacity improvement and suggested that with some other improvements along the lines, they might be able to  increase it further to 100%.

Electrification, reviving old ROW, and trolley/streetcar services are all very good ideas. There are some very nice little towns left, but they lack the density to make public transport work in many places. They might want to massively rezone the area around the stations and try to build more dense communities around them.

Nassau County is built fairly densely, especially along the LIRR lines. And in this case, a lower density is needed because you just have to get everyone to the LIRR station, rather than getting them to various dispersed destinations. Even having smaller dispersed parking lots along the feeder trolley lines is better than the huge central parking lots at LIRR stations. And then the parking lot land could be put to good use, like the aforementioned rezoning.
gotcha - thanks for elaborating