You can actually figure it out from the monthly performance reports (contribution divided by contribution per passenger mile) and its reported quarterly in the FRAs Rail Service Metrics (which they've been publishing again after a lengthy delay for some reason).Until Amtrak is required to publish each train's & route's revenue passenger miles instead of just overall RPMs one cannot tell how each route is doing. For example the Downeasters appear to carry mostly passengers from Maine to Boston north so its RPMs per train mile might be higher than another train's RPMs. An extreme example is the Crescent's RPM ATL <> New Orleans which by all measures is rather low.
IMO the number of passengers on a train or service has no meaning.