If I remember correctly, Harriman lot was expanded just a few years ago. I havent been up that way during the week, recently, to see how much the lot is filled. It seems to stretch halfway to Woodbury Commons now. Also, I believe the Salisbury Mills station lot does not have room to expand in it's current location, but I haven't been there in a while either.
I agree that double track could certainly turn this into a much better railroad. Reverse evening peak is not possible (not unlike the Gladstone Branch) at this time because of sidings. I'm sure they would be willing to work around a single track Moodna and Otisville tunnel to save a lot of money. If NS ever does come in, things will only get worse.
I think it may be possible that NJ Transit might not be the perfect, infallible organization that most people assume it is.