Hi Matt,
As Drew notes and his picture of the bare Details West Log Bunks show even for pulp wood which is fairly light, I would think, the Log Bunks still had to be very stout indeed and those bunks are all Steal too and notice how much heavier they are where they curve up from the base to withstand the outward push.
The question that determines the type of Log car used is what kind of timber are you planning on transporting?
A standard flat car, even with all the stake pockets filled with solid 4"x5" stakes would hardly be a match [NPI] for a single large size tree as they used to log out here in the West & North West. Instead, heavy chains and Pillow Blocks [> wedges] were used to wedge the chain under the log and between it and the floor of the flat car to prevent them from rolling. Short 26' Chanel Frame Steal Log Cars were also used along with the same method to hold three or four smaller trees of about 3' in Diameter to maintain the position of the logs while transporting them to the Saw Mill.
Now, if your hauling much smaller trees and don't have your Flat Car piled high with them then the either the 4" x 5" Stakes, I use flat tooth Picks or possibly wood match sticks shaved down a bit or the metal Log bunks should work fine.
Show us what you come up with, Ok. I can post some pictures too but only of the Old time stuff.