Longer trains absolutely make more sense economically, as the marginal cost of adding cars to a train is low. The problem for Amtrak is it doesn't have enough equipment to do this even in normal times, and now it apparently sidelined a bunch of its cars during the pandemic.
In fact, Amtrak ran much longer trains before the heritage cars were phased out in the early '90s. I can remember seeing the Crescent running with 14-15 cars in that era, compared with the 9-car sets that were standard in the pre-pandemic years, and the Lake Shore used to run with 17-18 cars west of Albany.
So a lot of the constraints on Amtrak's long-distance service are a result of equipment decisions made in the mid-90s. They've adapted to these limits by using revenue-management (read sharply higher fares, especially for sleeper space), but the reality is that they have the capacity to carry many fewer passengers than they had 30 years ago.
That's why I'm hoping a good chunk of the new infrastructure funding will get used for new cars for both short- and long-distance services -- not just to replace existing cars but to expand the fleet. Without that, there won't be much latitude to grow existing services, let alone expand the network.