That's the key. In my last game, I endded up making over $900,000,000 a quarter carrying loose cargo (non-contract) not counting the income from my cargo only subsidiary. Another way to make money on high demand cargo routes is to use combi aircraft with cargo fore and passengers aft on the main deck. One of my favorites is the 737-200C. I ended up with more than 150 of em!
"When a man runs on railroads over half of his lifetime he is fit for nothing else-and at times he don't know that."