The 20min rule is not appropriate for freight, and it could usefully be changed. Commodities (coal, iron ore, oil and timber) are not perishable; the time allowed for their transport should either be drastically increased, or exempted from the rule altogether.
This would allow long distance haulage of these in long, less frequent freight trains.
I think this small mod would positively improve the gameplay. (This would enable the use of the many large freight locos that that are now available. I have not so far found any useful application for them.)