The advantage of the double door cars is that fork lifts could get inside to help with the loading or unloading process. With single door cars there was not the same room and you might have to use a lot more manual labor, maybe even loading the boards individually. If you do spot a single door car, I would suggest you select one with a 10' wide door, not a 6 footer.
Quite a few of the double door boxcars were originally built for carrying automobiles. Then the autorack cars took over the auto business more or or less completely, and the lumber industry enthusiastically adopted the old auto boxes. Bulkhead flat cars became the next great step forward, only to be cast aside in favor of the centerbeam.
To qualify my answer, though, I won't claim to be an authority.
John