In the Carter administration cutback plan, the DOT was ordered by Congress to look into this. As anyone who thinks about it can guess, the incremental cost of adding coach seats was really low and sleeping berths were low. That study was buried in the appendices.Overall, isn't the way to cut losses on LD trains to simply make them longer and sell a lot more seats and rooms?
The LD trains have a huge amount of overhead (stations, locomotive maintenance, depreciation, etc.). So as long as the marginal cost of adding a car is less than the revenues that Amtrak gets from it, shouldn't Amtrak just sell as many seats and rooms as possible?
Isn't this how freight RRs make money: by having long, long trains?
As I've posted before here, low fares and big trains with lots of coach seats is how the PDX<>SEA corridor survived. Each of the three railways in the pool kept a reserve of older coaches handy for peaks because fares remained low year round (see attached mid-1960's UP spares in the Portland yard of the Pullman Company). When Amtrak was set up in 1970/71 the DOT wanted to kill that route because the cities on it were too small, ignoring the ridership and the drag created by the connecting SP Cascade going tri-weekly. Then, as I've also posted, Amtrak was required politically to hold a rate umbrella over the bus companies.
Of course, Amtrak can't do much of this because they use their superannuated cars in regular service.