Additionally, all the land abutting the railroad right of way is public land, ie Scarsdale (incidentally, parking lot south of the train station, which is Eastchester's was leased to them by the railroad). So is the land in WP public land.
More details on that matter
1. Yes the ROW is 4 tracks wide all the way to NWP.
2. There were once 3 tracks in White Plains station (the pre-high platform station, not the one there now). The third track was on the northbound side. I have heard people swear up and down on this forum that there were four but I have been told (years ago) by multiple NYC men that there were never four, only three. That third track went all the way to NWP. It was gone by the 60s, I don't know more details than that. The third track did remain for some distance south from NWP and I recall it being used for pickup and setout of strings of freight cars
3. There were four (Two middle sidings) at Crestwood. One continued north about a mile beyond Crestwood. I personally remember those being there in the 60s.
4. When the third track was put in north from Mt. Vernon west,for some strange reason they did not use the previously graded ROW and the pre-existing bridge abutments crossing the Bronx River south of Bronxville. I never understood why.
Sure, it's possible -- at least engineering-wise -- to add more track anywhere in there, but a monumental and costly project to be sure!!