@Stonelouse, placing signals outside of the loop will essentially cause traffic to back up on the single line section, if the loop is occupied, which you don’t want. You always want the single line to remain clear unless there’s a slot for it to go to in the next passing loop.
Consider this scenario, you have a train coming along the line from direction A (on your diagram), as well as a train coming from direction F. However, you also have trains stopped at both the platforms in the loop. As you can probably figure, all your trains are now in stalemate and neither can move unless you reverse the direction of either the trains at directions A and F to clear the section.
In a real world situation you have intelligent drivers who generally have block tokens that give them priority to enter single line sections. In Train Fever, you don’t have that luxury.
Hope this helps 🙂