G'Day, here are some guidelines for N scale model trains based on my experiences with Cubify Design and printing through Shapeways
First you really need to determine what sort of material you are going to use for your model, as this sets many of the parameters. I focus on two types:
White Strong and Flexible (WSF) is the cheaper model, has a rougher surface finish, and about half the resolution. It is great for making things like underframes (like on my Z tram) for inside models. I also use it for things like buffers ad the surface texture helps to give a bit of a wood like feel.
Frosted Ultra Detail (FUD) is the go to material for anything requiring detail. It is a bit more brittle, has a great finish and takes detail at 0.1x0.1mm.
Shapeways has a guide to features, which is occasionally changed. Key elements for FUD as of Feb 2014 are:
Minimum supported wall thickness 0.3mm (a supported wall has other walls on two or more sides).
Minimum unsupported wall 0.6mm (for wagon sides)
Minimum supported wire 0.6mm (useful for stays and break gear where it has support on both ends)
Minimum unsupported wire 0.8 mm
Minimum emboss detail 0.1x0.1mm (like a rivet)
Minimum engraved detail 0.1x0.1 (for grooves in planking)
Now here are some practical guidelines. In general I have found a tendency too go to fine when designing, because you can.... when it is printed and painted slightly chunkier detail actually looks better. The best way to do this is to emboss out to at least 0.2 and 0.3 is better.
Rivets. I use 0.15 x0.15. I found that sometime 0.1x0.1 wouldn't print, and 0.2 x0.2 was a little big. Using 0.15x0.15 appears to solve it.
Model Walls. If you are going to cast the finished model then a minimum of 0.8mm for shorter walls (say under 1cm in height) is ok, but for coaches, locomotives, wagons it is best to aim for 1mm think walls, and more upto 2mm if you can to make a resilient model. So for the QN / QR models the alls are 0.8mm. For the end walls on the VFTY I went down to 0.7mm, however they have outside braces for strength.
Stakes. On the VFTY I have a 1.5mm x 1.6mm for the stake, which is pretty resilient, I "cut in" 02.mm from the edge and 0.4mm deep to give the U frame. technically 0.2mm is too thin for a "wall", however as it is attached to a decent feature it counts as embossed detail... a useful trick.
Embossed detail. I have fund that 0.1mm emboss is too thin, you loose the detail. It is best to emboss out at least 0.2mm.
Planking gaps. To model gaps between planks, use a width of 0.1mm but go for deeper cut of 0.2mm. Sometimes though this can be a bit "washed out" when printed so a width of 0.15mm may be better (I'm still testing this).
I have also been experimenting with the use of Micro-Trains 2001 couplers, these allow the riding height of the wagon to be lowered to a more correct height. The problem though, especially with bogies, is having enough wheel clearance underneath the wagon. After a bit of trial and error I have come up with the foillowing:
To give enough clearance there needs to be 1.7mm from the bogie bolster (where the bogie touches the underframe). You mount the 2001 coupler 1mm "down" from the bogie bolster. The coupler pad needs to be 6mm deep and 5.2mm wide. The coupler hole is 3.6mm in from the end of the wagon and 1mm in diameter.
You can see how I have cut away in the underframe to give the bogie room to swing, remember though you need 0.3mm "above" this for the floor (it is a "supported wall"). Therefore aim to have at least a 2mm thick "floor" from the bolster to the floor on the inside of the wagon. A normal Micro-Trains bogie pin is 2.7mm though, so best to try and have a total thickness at the bolster point of 3mm, to allow for the bogie pin hole to be 1.9mm diameter and 2.7mm deep; otherwise the pin will need to be trimmed, or the bogie pin hole will go through into the floor of the wagon.
Hope this is useful for other designers, please feel free to ask other questions.