JoeM, welcome to FT!
I travel BOS-NYP-BOS quite frequently, and it's very busy. Even though I live closer to RTE, I backtrack to South Station to get a good seat. Weekend trains are usually booked full, as there are only a handful of trains and they see a lot of use by non-business travelers (i.e. families - go for the Quiet Car!).
Even First Class (the costs and benefits of which are discussed at length in other threads on this forum) is often sold out. This morning I was on 2190 (the 6:20am from NYP to BOS) and it was pretty busy, though there were seats available in each car. The busiest weekday trains northbound seem to be the 8am, and 3, 4, and 5pm departures from NYP to BOS. Southbound from BOS, it's the 7 and 8am, and 4 and 5PM trains that are the busiest. I'd really like to see a 6pm hour BOS-NYP train reinstated, as we had a few years ago.
I'm sure those that ride the NYP-WAS side can comment on that part of the route.