Demurrage charges figure in a lot that's been written about activities of Station Agents and Freight Agents: chasing customers to pay, debating legalities and fault when incurred etc. I believe they were primarily applied to RR-owned cars, but I have heard of exceptions. Sometimes they were an incentive for a business or an industry to create a fleet of privately owned cars, if the commodity was slow to load or unload, or was sold out of the car rather than unloaded into a warehouse. I expect demurrage was also behind RRs building and staffing Freight Houses. With RR staff and storage, customers didn't have to organize a crew and transport the first morning a car was available.

Demurrage appears to have been relatively standardized across the US by 1910. The short form is that between 1939 and 1975, customers had two days to load or unload, starting at the first 7 AM after the car was placed. Rules and charges were regulated by the ICC, but it appears the rules weren't as ironclad and nationwide as the underlying freight rates. The best overview I've found is a 2019 Federal Register article:

Search through the document for "1910" to go straight to the historical outline.