I surrender. I've spent the better part of the evening searching for some written info on how to safely open a 1948 Lionel #2628 Heavyweight car (tuscan, not the mythical green.) Thus far, I have not found anything helpful. It may be out there, but I can't find it!
I have removed all the screws that are visible, including 6 (2 on each end, 2 in the center) that appear to hold the frame to the shell, a couple of screws in the light sockets which have now released their lights, and one in a vestibule roof (under the vestibule.) The other vestibule doesn't seem to have a screw, but rather something like a post sticking down, of the type where you squeeze the sides, but it seems to be rigidly attached.
The shell remains FIRMLY attached to the sheet metal frame near the center, while the frame ends do flex downward a wee bit, just enough to convince me they are no longer attached to the shell. The vestibules on both ends are loose, the one that held a screw being much looser than the other, but neither slides out of the shell far enough to be removed.
There is evidence that the previous owner got to the same point as I did, the evidence being the wrinkled frame edge along the center bottom of the car. This appears to have been caused by his screwdriver trying to pry the sides of the shell away from the frame to release imagined tabs that might be holding things in place. But seeing the damage that he caused, I am reluctant to follow in his footsteps - the paint on these old Bakelite shells is notorious for flaking and chipping, and this one is still in pretty good shape.
Any thoughts? BTW, not 100% certain of the year, but it is original PW.
UPDATE: The shell is removed! The crinkled edge of the frame where the previous owner had damaged it was binding against the shell. He had pried one end hard to try to get it loose and actually started to bend the frame in half, so crinkled edges resulted at the bend point. 10 minutes with my press and a pair of flat nose pliers to straighten this and many other frame bends, and all is fixed. The paint had also stuck along the sides where the frame met the shell making removal even more dramatic. Bottom line: Removing 6 screws in the frame should have been all that was needed.
Thanks for listening! Now on to repairing damaged shell screw holes, cleaning, lubing, replacing wiring and replacing window celluloids. A labor of love! 
George
