I've just updated these Clamshell parts to improve the fit when they're together on the flatcar. Please take a look at my other models for the HO NASA Shuttle SRB train. Walthers produce a 66' four-truck, heavy-duty flatcar which is virtually the same design as used by NASA. An extra deck will need to be added using 0.040" plasticard to raise the covers to the correct height. Please note, the parts will need some preparation before painting. My thanks to Morgan Paul for photos of his finished models.
If you require decals to complete the model, please contact me. I have also created a 'one-piece' version for customers who do not want or need to model an opening version.
A fellow designer, HODiver, has created the Heavy-Duty 8-axle flatcar deck & truck-mounts, if you are having trouble obtaining the Walthers version. Please see the links below and visit his shop.
https://www.shapeways.com/product/U742EQ92F/66-heavy-duty-flat-car-walthers-design?li=search-results-1&optionId=58755784
https://www.shapeways.com/product/EBX79WBW2/truck-mounts-for-the-hd-flat-car?key=1e99a94c1c131a4980b9dcdc94a30970