If you are running a train with longer rolling stock or a large locomotive you will need to run larger radius curves to insure smooth train operation. Also if you are running concentric loops in your layout you will need to run larger radius curves. This will eliminate your 11-1/4 Radius Terminal Rerailer track from your loop so you need to use the 10 Straight Terminal Rerailer. This also gives you more flexibility with locating your power connections for your layout.