In the Northern Pacific files, in the President's Subject Files, the Chief Engineering Files and the Mechanical Department files are found a report, Report of Committee on Open Top Dump Cars, this report commissioned in 1929, updated in 1935, and then again 1939 - listed as the coal traffic between the Twin Ports and the Twin Cities to be the #1 determinant in open top car purchases on the NP.
Given the time that most cars remain in service, the 1939 update would pretty much cover this sites time frame.
To answer another post, No Tim, I did not confuse the Ore cars with the others. I believe you are wrong there. Of course, you are welcome to back your assertion with source data.
Of Jim Betz,
Jim - If you are a member of the GNRHS (I believe you are) please check GNRHS reference sheet 248 of December 1996. It (in a very nice article by Martin Evoy III) does list operations and preferences) so no need to theorize. There is plenty of factual data available for those that care to research.
In the article, He (Martin) does list the hand unloading that Dennis Storzek references in a more current post.
I also have (copies of letters) letters from the NP's General Managers office to shipping agents in Duluth/Superior regarding coal loads that were refused when they came in the wrong type of car, and these are some rather strongly worded letters as railroaders tend to speak. Yes, the consignee was free to specify the car type.
We seem to cover this topic of coal and gons and hoppers every several years. Or like September 2015 - Seasonal coal loadings was the topic.
Ben Hom (I believe it was) commented once about - Doesn't anyone check the files?
Of clamshell bucket unloading of gons, see in the Photo area my postings from April 9. 2008 - Small Town Coal Operations. There is a crane with clamshell unloading an NP composite gon, among the photos. Date of photos is early 1930's.
Of other coal postings - see postings 111293, 65630, and 65627.
Jim Dick - St. Paul