So, feeling frustrated, I set them aside for several months. However, the recent blizzard left me stuck in the basement for three days in a row with nothing to do but work on projects, so I dug this out again. I realized that extensive bracing would be required for the sides to work, and that meant a detailed interior would be impossible. Oh well, for my first NMRA scratchbuilt car maybe that is for the best. I used very strong 1/4" square styrene along the bottoms and 1/8" square styrene in the corners and on top. Piece by piece, I stiffened the edges and slowly but surely the parts straightened out.
The corners were especially troublesome because I couldn't brace both vertical edges evenly. No matter what I did, there were some bulges. So, I glued it all up and for those areas I flowed in more MEK and carefully used my fingernail to hold the joint closed until it fully cured. Thank goodness I had a TV on my workbench, as I sometimes had to hold joints for 10 minutes or more. I am sure that they make special clamps to hold corners such as this, but I was afraid of using something that would force the soft plastic to squeeze out and get somewhere that it didn't belong.
For the joints where clothespins could be used, I took advantage of them. They are one of the cheapest sources of good clamps, and along with binder clips should be in every person's workbench. Plus, MEK has no effect on the wood. As the day went on, my caboose slowly began to look more and more like it was supposed to. However, due to the warp on the edges of the ends it was turning into a parallelogram instead of a rectangle. After some thought, I set it up on a piece of graph paper (which I assume has straight, perpendicular lines) and used a clamp to gently force it into a rectangle shape. After lining it up on the paper, I added corner stiffener braces and let it cure for the rest of the day.
I used a straightedge along the roof parallel with the long direction and made sure the two ends and the two braces touched the straight edge all along the curve. One brace was slightly too short and I added a thin strip along it to raise it up. One brace had a slight bump that I carefully filed down. Then, I took some thin 0.020" scribed styrene and measured it up for the roof. I glued it scribed side down, which looks realistic from the inside (though it will probably be impossible to see) and bent easier because of the grooved marks. Then, another layer of 0.040" styrene went on top to make a strong roof. I was careful to mark center lines and make sure both layers went on straight, as correcting it later would be difficult.
The side joints where the two layers of the roof met the top of the caboose were also a little tricky. I flooded the joints with MEK, held the edge of the caboose roof against my workbench, and then applied strong pressure to hold it there for 15 minutes at a time to make sure it set. In G scale I would have just used blue painter's tape to hold the joint, but even a little smudging is obvious on a caboose this small. I also dug out some Testors white putty to work into the cracks. I rarely use it and don't have the finesse of an expert model builder, but it did fill the gaps. I have heard that Squadron brand filler works better, so I might try that in the future. I still need to sand this smooth.
Finally, I worked on the chassis. When I first built it, the outer two frame members that had the axle boxes bolted onto them were built strictly to the plans. I discovered two problems with that: (a) the wheelsets wouldn't fit between them and (b) the wheel flanges interfered with the inner frame members. To get around this I first drilled out the rear of the axle boxes with a #30 bit which allowed the full point of the axle end to go in easily and spin. Then, I cut away the offending portions of the inner frame members where the wheel flanges rubbed. You can see in the picture the missing parts. I may glue up some thinner pieces to try and fill in the gaps, but once it is all painted black it might not be noticeable.