In the near future, limited run, hand-built models in N-scale and Z-scale will be made of various resins which come out of advanced 3D printing Rapid Prototyping systems.
Scale fidelity will be better than what has been available in brass since most of the models will be painted and "assembled" rather than stamped, etched, machined, bent and soldered together by hand like brass models are.
I would venture to state that they will also be designed, manufactured, and assembled here in the USA and Canada.
Some conversion kits are already available through entrepreneurs working with Shapeways, but, the Shapeways business model and equipment/materials used have severe limitations. Some conversions are available which have been produced using resin casting technology, but that also has severe limitations.
However, the really fine, ultra-quality models will be made on EnvisionTEC (or similar) systems which have an ultimate Z axis resolution of .0006" (less than 1/10th of a scale 1/160th inch), and produce no "layering" like the Shapeways systems.
I believe that a customer probably wants to feel "heft" in his hand when paying $500 to $1,200 per model, so at first, North American custom N-scale model railroad manufactures will produce a hybrid that is part metal, and part resin.
A short while after either the success or failure of those attempts, nearly pure resin detailed engines and cars will be produced with metal chassis, and some lost wax and etched parts where appropriate. The detailing will be exquisite and far above what is currently accepted for either injection molded or hand made brass engines, with things such as completely detailed backheads and under-engine suspension, braking and equalization details extant.
Part of the big problem with the brass importers is that they have to guess what will sell well. They've attacked that problem by not ordering a first run of the proposed project from their Asian manufacturers until enough committed pre-orders have been taken. However, as the price of the end product continued to rise, the pre-orders started to wane. So today, new brass engines, cars and other models manufactured in Asia are becoming few and far between in N-scale, because the majority of us N-scalers are not willing to pay upwards from $1,800 for a brass engine.
With RP 3D printing technology, I believe that even the most complex of N-scale steam engines could be produced for less than $1,000...and possibly for up to 20% less, with diesel motive power being produced for just a little more than what we pay for Kato, Intermountain or Atlas diesels. However, they will be much better detailed and more simple to produce since they will not be encumbered with the restrictions of the injection molding process.
Although we are in a "transition era" ourselves as far as N-scale manufacturing is concerned, I am looking forward to the near future when dedicated N-scale manufacturing entrepreneurs become more mature and confident enough to step up to the plate and offer either complete kits (with shell, details, chassis and motor), or complete, finished models of road-specific engines many of us want, but cannot expect a major manufacturer to ever produce.
Cheers!
Bob Gilmore