In one of its fleet plans, Amtrak went on and on about this question, noting that the on-again-off-off-off-again practice had wiped out the domestic supplier base.
The fleet plan said the company had talked to several likely bidders, who told them than an order needed to be about 100 units a year to gain the better prices from economies of scale. The fleet plan went on with Amtrak planning to order 100 single-level cars for iirc 6 or 7 years, and starting a year later, 100 bi-level cars for 6 years or so. The orders were going to be strictly 1:1 replacements, no expansion included.
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In a normal world, with 70 cars in hand, and 60 in the bush, Congress might put in a few more pennies now to order more sleepers, more bag-dorms, and more baggage cars.
I agree that economies of scale are desirable, and in a perfect world there would be enough orders from enough customers (more than just Amtrak) to achieve them. In the current environment I'd rather spend, say, 20% more per unit on my fifteen a year to get reliable equipment delivered on time by an experienced supplier with skilled employees than the current Viewliner charlie-foxtrot. But, as you correctly point out, as long as Amtrak's equipment budget is wholly dependent upon Congressional largesse, that may be difficult to achieve [my scenario did specify that I was 'Amtrak czar', with the sufficient funding to make a difference]. We may have to go back to the days of Lyndon Johnson spreading Apollo program patronage around all 50 states....
I don't think the CAF order (or the N-S order) would do much for Congress at this point. Both have been rather humiliating messes: CAF is running on seven years with only a batch of baggage cars (and a single diner) delivered while the N-S order failed crash testing (if only by a hair, per some rumors).
Additionally, my understanding is
not that Amtrak intended to/planned to/wanted to buy 200 cars: They had options on another 70 (IIRC it was 35 more bags and 15 more each of diners, bag-dorms, and sleepers). However while I could speculate on the utility of more sleepers and bag-dorms, I don't think anyone here could think of a use for more than a small number of those diners (let alone 35 baggage cars). I think it was jis who noted that the option was a planning tool that
might get used to shake a few spare cars out if we got lucky; as it was, it was used to switch some bags and bag-dorms within the order as things dragged on (I suspect to finish replacing some of the Heritage bags sooner seeing as I don't think there's a bag-dorm in sight).
Moreover I would argue that CAF is to blame for that option being unusable: IIRC the option expired
years ago (it had already expired when I approached VHSR about talking to the VA DRPT about exercising some slots on it) and CAF hasn't even delivered a single sleeper. If they had been able to churn out the sleepers first there's a chance that Amtrak could have "counted beans" and gotten an RRIF loan to thrown a few sleepers or bag-dorms on the back end (while the baggage cars were coming down the pike) but as it stands that's not likely.
Hindsight being 20/20, the Alstom-Bombardier bid was probably the smarter option...but my understanding is that
their bid was simply too expensive by comparison for Amtrak to justify. If more money were to come available, I'd be
mighty inclined to go with Siemens at this point (seeing as a Siemens-based order from AAF has gone from conception to delivery and, knock on wood, start of service entirely within the timeframe it's taken CAF to deliver a single sleeper).
Edit: To be clear, Amtrak's latest fleet strategy plans have all presumed something of a boom-bust scenario in this respect. Part of the problem, IMHO, is the "split fleet" situation: Amtrak basically has three fleets (Superliner/MSBL, Amfleet/LDSL, and Acela). The Acela split is somewhat unavoidable due to the nature of that stuff (orders for that will always be in batches, and the new federal standards don't require Amtrak to reinvent the wheel on new equipment there) but the single-level/bilevel split is a very real problem for a
bunch of reasons (not least being that a new railcar either "belongs" to the extended NEC/NYP-based portion of the network or it doesn't, but also because Amtrak can't just keep pumping out orders for cars in single body types with different, particularly modular, interiors).