Most of the time I use road lines for freight. When I create a new freight line, I always start with load if available. As soon as the production has increased and the first trucks get fully loaded, I switch to full load. Then from time to time I check whether there are freight items waiting for trucks or trucks waiting to get loaded and change the number of trucks on that line accordingly.
I guess you could do a similar thing with trains. Just make sure they don’t have too many cars so the waiting time is not too long when waiting for full load.