The technology to do seat selection is pretty straightforward,and already exists, whether it is a positive or negative is an entirely different story. It would benefit some (myself included since I tend to book pretty far in advance) but it might add cost and tension that exceed its benefit.
Yes, it would definitely benefit me as well but funny as I think it causes LESS tension, at least once at the station. People always seem so anxious about making sure they get on the train (and get a good seat). I think if we had assigned seats people in the waiting area wouldn't be so pushy. But that's JMO!
Assigned seats are a very different animal from passenger selected seats.
With the exception of last minute reservations, Amtrak obviously knows days (or minutes, on trains where seat
assignments are made at boarding) prior to departure who is travelling where, and seating could thus be assigned accordingly so that all parties sit together, nobody must hunt for an available seat or be asked to move mid-trip, etc. This would also eliminate the problem of people trying to be first in line to board, as there would be no need.
On the other hand, allowing passengers to
select seats (ahead of actual boarding) is problematic and far more complicated than just a technological issue. Permitting (particularly early-booking) persons free reign to select
any available seat on the train just won't work given the extent of intermediate-point business (it is much simpler for the airlines, which lack multiple stops). You would effectively block later reserving passengers from even making a reservation by creating a situation where there are plenty of unoccupied seats at every point, but no single seat available for the blocked passengers' entire journey. The train would become "sold out" when there was potentially ample inventory remaining.
What will work, of course, is offering passengers either a limited selection of seats from which to select or else limiting the number of passengers who can make seat reservations at all. As you have noted, single travelers can block parties of two or more persons from sitting together, but this may be avoided by only allowing (or assigning) single passengers to select seats next to another single passenger. Parties of two might be offered only pairs of seats; They would not be given the option to book two window seats, for instance. Seat selection also works if you have only a (very) small number of passengers pre-selecting seats, such as those who have paid for the privilege (which sounds like a winning idea, frankly), as the numbers of seats potentially "blocked" is small enough you can probably fill them with odd numbered parties or, at worst, cause only minimal seat switching.
On crowded trains with lots of stops, who is going to chase people out of seats? The opportunity for confrontation is too great, sadly we don't live in a particularly civil society.
Nobody should have a need to chase people out of seats (with rare exception); That's why it is important to be careful how any seat selection program is implemented, and what makes such a feature far more complicated than it appears.