One of the answers to How rigorous are Amtrak in enforcing luggage rules? made me wonder how busy the Boston to New York train gets at the various points of day? Also, what is busier: weekdays versus weekends?
The Acela trains in particular can be quite crowded.
The Northeast Corridor Amtrak routes on the Washington to Boston corridor are easily the highest traffic routes in the US rail network. In fact, there are many that actually commute along these lines several times per week between the various cities. (I.E. living in NYC and working in Boston or Philadelphia).
I would say that, in my experience, weekdays, especially early morning and early evenings are the most crowded, but this can vary a lot depending on factors like holidays, schedules at various college towns along the route (less of a factor for Acela), etc.
Regardless, if your concern is specifically with regard to cargo space, as noted in your other question, I find that this has little to no correlation with crowds on the Northeast corridor. As noted, a great many of these travelers are commuters or day trippers who carry only a simple briefcase or backpack.