This is my train of thought in regards to modern rail traffic as it applies to the lumber industry:
Bulkhead and center beam flats are used by lumber mills to ship dimensional lumber, while All-Doors and the like are typically used my molding companies to ship finished wood products, such as doors, etc.
In other words, the sawmill cuts the logs and ships the dimensional lumber to both the Molding Co and the Lumber Wholesaler on flat cars. The Molding Co turns the dimensional lumber into doors, moldings, etc., and then ships those products to the Lumber wholesaler in boxcars.
No lumber mills In NJ, but the big lumber yards get their loads on centerbeam flats & box’s. Doors come in box’s as does moldings, cabinets, etc. My [2c]
I just got finished reading “The Modelers Guide to Logging Railroads” by Matt Coleman. All indicatons are in the latter years of logging, and shipments coming from big mills, 90% of cut lumber from the saw mill was kiln dried and shipped out in boxcars. So it depends on the era that you are modeling that will determine how much cut lumber is shipped from the mill on open cars that get wet. However, it appears that all cut lumber consisting of hardwoods were/are shipped out in boxcars.
I recently followed a bit of on-line discussion regarding the all-door boxcars. My recollection is that they aren’t used much these days–not like back in the ‘80’s or so–something about the cars being too delicate for the job. I think you would use 50’ and 60’ double doors instead.
Don’t forget about the woodchip cars leaving your mill.
You should also consider dealing with laminated structural beams and maybe also built-up trusses.
