This still begs the question. Why doesn't Amtrak offer seat assignments when you book on line? Every airline and corridor or LD train in Europe does. Yes Amtrak has different car types for coach, but think of all the airline seat configurations just on one plane, like B737.
While there are many configurations of a B737, airlines know in advance what type of aircraft will fly on a specific route on a specific day. So if a B737-500 is scheduled, it will show a B737-500 seating configuration. If a B737-800 is scheduled, it will show a B737-800 configuration. If a B747-SP is scheduled, a B747-SP will be shown. If a B747-400 is scheduled, a B747-400 will be shown.
Can you imagine how confusing it would be to be shown he seating charts for "train 98, 1st car" "train 98, 2nd car", ... "train 98, 6th car". Then if the yard crew hooks up the train "wrong", putting the "3rd car" in the "5th car" spot, I'm sure passengers would complain. And how about if they have to substitute a car, like an 18 seat BC car for a 60 seat BC car. I'm sure you'll hear complaints from the couple who selected seats 45 & 46!