There's no reason why you couldn't as long as you design the parts to the characteristics of the material. For example plastic will cool at different rates depending on its thickness and at which point the material in that area will begin to set. So long flat things tend to warp unless properly supported, thick sections will become slightly concave.
I've used it mainly for detailing parts on G scale stuff; domes, smokebox doors, funnels etc. I have successfully made an LGB type motorblock and moving parts such as pistons and connecting rods. In OO and OO9 I've built chassis and bodies with minimum fuss. There's a chap who turns up with the Gauge 1 society at a lot of the shows who has whole trains of 3D printed 1840's standard gauge stock. The reason I've never used it whole items of G scale stock is that it has often seemed a lot cheaper, easier and quicker to model bulk of them from something else. Even where I have designed the whole thing in CAD first. There's the added benefit that you don't have to do as much prep work to the surface too.
I've done all my printing through
Shapeways and
3D Print UK which have a number of materials available and several design guides. I'm currently researching into buying my own machine, Creality seem to be delivering the most bang for your buck at the moment. I have put off buying my own so far as the technology available in home printers (and the price for a unit) is changing so rapidly. All my design at home is done in Autodesk as I received a free license when I was a student.
As with all manufactured items, its success will depend a lot upon the design. Be prepared to sink a lot of hours into thoroughly learning a CAD program before you commit to a printer if you want to print your own designs.