We use the following strategies for train loading:
For passenger trains, we don't usually enforce a maximum loading.
We usually combine passenger and mail service, in a 2:1 ratio (2
passenger cars for each mail car). We don't want the train to be
waiting for mail when it is full of passengers, so we don't set the
"load" switch for passenger/mail trains.
We run "unit trains" for all commodities. With unit trains, setting
the "load" switch has two beneficial effects:
The train makes the maximum amount of money per trip
The industry is encouraged to produce the maximum amount of the
commodity being transported
Watch out for one thing when setting the "load" switch: Some
locomotives carry passengers or mail. If you are pulling a coal
train with such a locomotive, your train will never be fully loaded,
because there are no passengers or mail at most coal mines!
