Originally Posted by
Rick Merrill
I think I see: it is the 'coach' that is reserved, not the 'seat'.
I'd note that there are children who were born in this thread's dormancy who are now licensed to drive. A few things have changed since then.
Reserved vs. unreserved basically boils down to "best effort to ensure a seat is available for every ticket sold" vs. "no such effort".
In the 16+ years since the last posts, nearly all of the unreserved trains have gone away: I believe the only ones left are the Keystones west of Philadelphia and the Springfield Shuttles between New Haven and Springfield. Both of those are in a gray area between commuter rail and Intercity rail.
With the rise of internet purchasing and especially mobile apps, the NEC services seem to be moving toward assigned seating.
On longer distance trains, conductors will often try to assign passengers to coaches by destination. If nothing else, having all passengers who will be reboarding between midnight and 6am in the same coach minimizes the number of passengers who get woken up or jostled by another passenger getting off the train.
Sleeper accomodations are auto-assigned at ticketing. I forget if it's even possible to change the assignment, though in principle the Acela selection system should be applicable.