Sorry for the delay in replying but work and real life gets in the way sometimes!
Tight fit: caused by the chairs being drawn to fit my stock of 20+ year old rail. Current supplies seem to be of a slightly different cross-section; possibly due to worn tools at the rail rolling factory? The 3D prints will also vary; Shapeways' tolerances are published on their website.
You may get a slightly better fit if you set Templot to measure the angle as CLM rather than RAM. CLM is what the prototype used so that's what the chairs were drawn as. Bear in mind that an under- (or over-) sized print will affect the spacing between the rails. How is the angle of your filing jig measured?
The FUD acrylic material is brittle but is the best material offered by Shapeways for reproducing the detail; indeed it is the only material available which will reproduce some of these small components at all. It is totally different to the styrene used by modellers and is best sawn (with a piercing or fine razor saw) rather than cut with a knife. If you soak the items in white spirit for a while then not only will the oil and wax used during printing be removed but the surface will, according to the Shapeways forum, turn foggy/frosty/hazy making the components slightly more visible. It's also worth marking the components, before removing from the sprue, with a permanent marker pen. Red for port (left); green for starboard (right) and black for the end nearest the toe of the turnout.
Currently (for the last year at least) on the drawing board are chairs with separate tapered keys in an attempt to ease some of the above the assembly issues.
Hope that helps.
Regards, Dave