I'll comment that the "preheat to 150 F -- TURN OFF -- put part on a flat plate (cookie sheet) for 10 min" is in Sunshine general instructions and it works.  My additional comment is that this works first time on thinner parts like car sides.  Thicker parts, like floors and one-piece molded roofs, probably need at least two of these treatments because each time the part relaxes a little but not completely.  

On occasion, I have used ACC to attach styrene stiffeners behind a boxcar side because the fitup still was not optimal even after this treatment.  Just bear in mind, the combined resin side + styrene stiffener may still exhibit a warp, depending on the relative size and orientation of this part.  You have to watch this technique, because those stiffeners might get in the way of, say, drilling grab irons, or weights that might already have been glued to the floor.  Ask me how I know.

On a related note, I ended up needing to glue large stiffeners to the floors of some Walthers ex-proto boxcars because of floor warping.  Worked, and added a tiny bit of weight.

Another related note -- a brass stiffener is much more stiff than a styrene one, but you run the risk of differential expansion between the brass and the resin.

Ron Merrick