There is also the cost of not using thicker lumber.
To explain further: The Northern Pacific had a large number of drop bottom gons and (in other than company coal hauling service) as usage cycled through the year -
Late fall into the early spring - commercial coal hauling.
Late spring into early summer - company service handling of gravel and ballast for roadbed repair or upgrades.
Summer - commercial usage of aggregates.
This in addition to the usual hauling of steel components, tractors, etc., however I am referring to summer commercial use hauling aggregates. The NP found a lot of damage to the doors when these cars returned, enough that it caused coal fines to leak to the point of consignees complaining.
They tracked this down to commercial operators dropping clamshell buckets from excessive heights when unloading and at times opening the buckets with larger materials, also from excessive heights when loading.
The response to this was the NP started then lining the bottoms of gon let out into commercial service with 6 x 12 rough timbers.
I will line some gons this way (a few) as I have the paperwork to verify this should questions happen,
of course with your railroad, YMMV
Jim Dick - Roseville, MN