We've sort of been talking about this over in the Siemens-Amfleet replacement thread, but I thought it would be worth a separate thread.
Questions:
1. Operating costs: When, if ever, could Amtrak add a coach or sleeper to a train and have fares pay for all the additional variable costs from adding that car? Put another way, when could an n+1 car train have a better farebox ratio than an n car train, so that Amtrak (or Amtrak and a state) could run a longer train on the same subsidy? I think the answer depends on:
a. How much does it cost to add a coach or sleeper to a particular day's train? How does this vary based on type of car, the number of cars that train usually has, and anything else?
b. How many additional passengers could Amtrak get, on average, if they added a coach or sleeper to every train that was about to sell out n weeks in advance, or that had sold out the previous year during a particular set of events along that route (college break, certain conventions, school vacations, sporting events, whatever)?
c. Whatever else occurs to you.
2. Capital costs: Are those times plentiful enough and scattered through the year at different times on different routes such that Amtrak could keep an extra ten, twenty, thirty cars in enough use to justify their maintenance, inspections, storage, and other costs of having them around? Justify could either mean "have additional fares cover increased costs of ownership" or, somewhat easier to get to, "additional ridership requires additional capital subsidy, but results in the same or reduced capital cost per passenger or per passenger mile".
I think my overall question is something like this: Could some trains carry more people for the same subsidy if they were longer some of the time? And I guess it's part of a bigger question that we sometimes talk around the edges of: Could Amtrak (or some similar national passenger train system in the US) get n times as many passenger miles without needed n times as much subsidy; would a system of longer and/or more frequent trains cost require less subsidy per passenger or passenger mile, and could it even be possible to get a lot more service with a lot more passengers without needed much more operating subsidy?
Go crazy!