Rob,
Aluminum was shipped in bulk in the 60s or 70s and would mostly have been loaded in gondolas. The MR article shows billets loaded on a centerbeam lumber car. Those cars were just begining to developed for lumber loading around 1970. The use of them for hauling aluminum billets didn't start until the 1990s as that design of car was becoming very popular at that time.
Charlie
I did find this video showing RECENT shipment of aluminum billets:
https://www.youtube.com/watch?v=TeosC0JPy3I
In 1955, Great Northern converted 14 of their 66000-66249 flat cars to aluminum billet loading by adding "racks". The outcome looks VERY roughly like a gon with pieces taken out of the sides.
There's a photo of two (one numbered 66015) in Scott R. Thompson's "Great Northern Equipment Color Pictorial Book Two-Freight Cars" on page 83. They are empty. Photo taken in Spokane, WA.
GN got into bulkhead flats not much later. I would expect those might have been used for aluminum transport.
There was/is a lot of aluminum produced in the Pacific Northwest.
Ed
The article in MR March 2017 re billets. I wish to model before 1975, so when did aluminum start to move in bulk + what might it have looked like in the 60's early 70's as a load.
Thanks in advance from down under
Rob