While I am sure that Amtrak has some general numbers and
delivery dates in mind, I assume that the actual numbers
and dates will only be set once contracts are negotiated
with the suppliers.

Obviously, if a supplier quotes a lower figure per car for
a larger order, Amtrak may be order more cars for the
same amount. And, different suppliers will have different
production capabilities, though, again, I would assume
that if a supplier gets a large-enough order, it would be
able to ramp up its production capacity.

And, once actual specifications are put out for bids, you
may even get some new suppliers interested. For example,
Vossloh, a major European motive power manufacturer has
long been trying to break into the U.S. market with a
diesel passenger locomotive based on a proven European
product.

http://www.vossloh-espana.com/cms/media/downloads/pdfs/flyer/Vossloh_Espana_EURO4000_passengers_us.pdf

(Detailed tech specs; uses EMD prime mover!)

I'm not completely up to date on this, but if I recall
correctly Vossloh was awarded a contract by Boston's MBTA,
but a competitor challenged the award under "build American"
provisions, and the order has been tied up in court. If I
remember this correctly Vossloh planned to bring in the first
two(?) prototype units from Europe, but then assemble the rest in
the U.S. with mostly American-built components. The
competitor (MPI?) claimed that the European-built prototypes
would violate American-built specifications.

Similarly, Siemens is also trying to break into the American
mainline market and has bought additional land adjoining its
California plant where light rail vehicles are assembled.
Yes, Siemens does also make diesel-electric locomotives, though
it is best known for straight electrics.

-- Ernest