What are you trying to simulate? Since the mainline begins at one end of the layout and ends at the other, you obviously won't be running through freights on the main as you work the switching sections. If you are simulating a part of the mainline wher the railroad would use passing sidings, you will need to use the switches to do that. If you don't need passing sidings there, just use the double cross over. Finally, if that section of mainline is not modeling a heavily traveled main, so that you can spot cars on the main line while working industries, you could perhaps save some space and expense by using some switches for crossovers & run arounds on the main line. If the third option is viable, it would seem to me that a run around set of cross overs on the mainline between the rolling mill and the ADM grain elevator would facilitate working the refinery tracks. In that case I would save some money by not bothering with a double crossover.