PMC,
There are economies of scale to be had. There are also other marketing areas to be thought about. Those of the Amusement industry, Tourist lines and replacement parts for existing locomotives to name a few.
With the right mix of parts, several locomotives could be produced with a minimum of inventory.
Yes, lost foam and other casting methods might work, but there is much more to it than that.
I would recommend that you read "Modern Locomotive Construction- 1892" by J. A. G. Meyer, reprinted by Lindsey Publications in 1994, ISBN No. 1-55918-144-3. This will give you an overview of the engineering problems involved in building a Steam Locomotive.
I would, if rebuilding a locomotive, look to a run of more than one as there are many tourist lines around the country as well as amusement parks and foreign lines that could be marketed to. I would not limit myself to a single gauge, either.... But that's me.
If I were to put in the time, effort, blood and sweat in meking a new locomotive, I would'nt quit with one.
Ther would be some, but not many, economies of scale as each would have to be assemble to order, unless you take the EMD track and offer limited options and varied colors.
(Yes, I DO have a Degree in Economics)
Rick