You don't actually have to buy a ticket. You can go through the motions of buying a ticket, but then not actually purchase it. The beginning part of the exercise will show which trains you can take and the availability of seats.
The one caveat is that sometimes short distance segments are blocked off to allow longer distance riders to have access to those seats. So your two stop journey might be sold out because Amtrak is saving those seats for people who might be riding halfway or more.
I would plan on it being full. Even if the website shows availability conductors or crew will most likely group passengers into certain cars to facilitate loading and unloading trains instead of letting passengers spread out. I've seen passengers jammed into 3 or 4 cars with one car completely empty.