David,
I don't think it's prototypical (think is the operative word, I'm not an expert on this) for these types of gondolas to carry coils as shown in your photo. I've seen photos of specialized coil cars that do carry coils pointed in that direction. And not to nit-pick, but your photo is of cable spools or reels, which is a much lighter load. But they certainly do look better that way.
Side to side rolling of these coils would be lethal - you are correct. My guess (and that's what it is) is that a fairly deep wooden "well" of 6"x6" boards and cribbing would be constructed and placed on the floor of the gondola so as to be wedged in place. The coil cars of the 1960s positioned coils perpendicular to the direction of travel.
Keep in mind though, the forward and backward jerking of trains is pretty dramatic (much worse than side to side), especially during switching maneuvers. That's why cabooses were typically cut out of switching exercises and one reason why cabooses were eliminated altogether - too dangerous to the occupants.
As for hauling coils using flat cars or flat beds vs gondolas, I don't know. I have read somewhere that coils were even shipped in boxcars prior to the advent of specialized coil cars. I know that I witnessed many trailer trucks carrying 2 coils aligned as described.
There seems to be a prototype for everything if you look hard enough. 
My original post was about the number of coils and their weight as assigned to our toy rail cars. The weight issues is an absolute. You can't load a 140,000 lbs. capacity gondola with 8 50,000 lbs. coils. 
Maybe someone has photos of the cribbing. I'd like to see that.
George
