Let's assume we're talking about an order for new single-level coaches and lounge/cafe cars, say full replacement plus how many more, uh, it depends. (But no more Superliners at this hypothetical time.)
With the NEC Regionals making positive operating results (often called "profits"), most replacements for the Amfleets should go first, not to replace, but to expand the fleet. Today any Regional that departs "sold out" is leaving money on the table. Adding cars to the existing Regionals' schedules should quickly increase the operating surplus to help pay for NEC infrastructure repair and upgrades for better future service.
As the added Regional cars fill with riders, Amtrak's nationwide ridership total will increase. That number is good for itself -- isn't the whole idea to allow more people to ride trains? It is also, as they say, huuuge when asking Congress for more money. "See? When we invest and expand, we can grow ridership and cut our losses."
So the NEC Regionals comprise an orchard full of low-hanging fruit.
Next step would add one or more cars to the existing system of LD routes. I've read that some of these trains sell out from time to time, and that ain't good. It's more money left on the table. Not sure how many cars could be added to the Lake Shore Ltd, already a very long train. Clearly the Lake Shore, Meteor, and Star need one or more cars. Maybe the Palmetto could fill another car.
The added revenue from more filled coaches could be very sweet. Imagine an Amtrak train that runs with a locomotive, bag car, cafe, and three coaches. Figure that the three coaches have already "paid for" the locomotive, bag car, and cafe. Then a fourth coach has low additional costs, chiefly fuel, maintenance, and the equipment charges. But it gets the train crew and some other costs "free", so more of the revenue drops down to the operating results.
Again, lots of low-hanging fruit ripe for plucking.
Well, nobody wants to run an empty coach ATL-New Orleans, tho the main Crescent segment NYC-D.C.-ATL could probably use another coach or two. But ATL will need new station facilities to unhitch cars that would be empty on the segment ATL- New Orleans. So before Amtrak can gain from added coaches on the Crescent, it waits for more money to be spent.
Adding another car or two to the Cardinal could help, but it really needs another train set so it can run daily. The PRIIA study forecast ridership would double from adding four more round trips a week, an easy 100,000+ added to Amtrak's total ridership. But we don't know how much would be needed for upgrades to the freights' right of way.
Next up, some of the state-supported trains could use another coach. Others will have better info, but I'd try another coach on the Pennsylvanian, the Adirondack, and the Vermonter, maybe on the Maple Leaf and the NYC-BUF Empire Service trains. The Ethan Allen might use another car when the extension to Burlington kicks in.
The Vermonter may need another coach when the upgrades are finished and run times slashed on the New Haven-Hartford-Springfield segment. For sure the Vermonter will need more cars when it gets extended to the big anchor city of Montreal. The Pennsylvanian will need equipment for another train set or two or three when frequencies are added NYC-Philly-Pittsburgh, after lots of money has been spent on the NS segment Harrisburg-PGH.
None of the trains mentioned above would require any new stations, commissaries, etc. Only mostly small upgrades to improve track capacity.
Replacing the Superliners on the Capitol Ltd with new single-level cars would take three train sets of them. Making the City of New Orleans into a single-level train too would require three, or more (the "Gulf Coaster"), train sets.
Single-level equipment is less efficient than the bi-levels, so costs would go up a bit on the Cap Ltd and the CONO. But their cannibalized equipment would be added to the Western bi-level LD trains which are desperate for equipment, often selling out in segments like Denver-Glenwood Springs-Grand Junction. This would be a stop gap until Congress can be persuaded to buy new bi-level cars. Congress will be more easily persuaded to do that if losses are down on the trains that have received added cars in the meantime.
Back to the Eastern trains, we need another train to Chicago, or more than one. Put the revived Broadway Ltd (or whatever name) top of the list. But it will cost big money for infrastructure. More frequencies of the Pennsylvanian would be part of that. But PGH-CLE-TOL-CHI is highly congested, and the hosts will surely demand substantial investment to create a new slot for a new (revived) train.
The best solution to CHI-East Coast service is to upgrade the CHI-TOL-CLE-PGH corridor to dedicated 110-mph or 125-mph track. At a cost of Billions.
All other added routes would also require paying for improvements to the infrastructure to handle additional trains. If we get another Stimulus, we can look to add a bunch of other corridor trains and the fabled Atlanta Day Train.
But that's another story.
