Amtrak is all reserved, but only the service, not the specific seats.
On long distance (LD) trains, passengers are assigned carriages depending on where they board/disembark, so as to minimise disruption for other passengers, and to minimise the needs for the train to stop multiple times at stations with short platforms or staging areas.
This was certainly the case when I travelled on the Adirondack between Montreal and New York. Montreal passengers were kept in 3 carriages at the front of the train, the other coaches being for people alighting at intermediate stops. This also aided in keeping passengers together for the passport checks at the US border.