A recent e-mail from Shapeways incuded a couple of videos on their production processes. Both are on the link below.
The first video, almost half way down the page, describes how they do metal objects from brass to gold. Basically it is a lost wax process or investment casting. They print in wax, make a mold from a plaster material (investment), pour liquid metal and do some finish work. Maybe not that useful to model RR, but cast brass may be useful for items too fragile when done in plastic.
The second video, near the bottom of the page, describes printing Strong and Flexible plastic (SFP). The detail is not as good as with Frosted Ultra Detail (FUD), but can be used for things where flexibility is more important than detail plastic which is more britttle.
https://www.shapeways.com/promotions/may-maker-days?li=home-promotions-mmd
George In Midcoast Maine, 'bout halfway up the Rockland branch