How do you loop tracks in Logic Pro?

Hover over the top right of your region and wait until you see a circle arrow icon like this:

Click and drag the circle arrow icon to the right and this will automatically loop your track or region in Logic Pro. Continue to drag to the area you'd like it to stop and then let go.

If you'd like a shortcut then you can press "L" whenever you have a region or track selected. Keep in mind, if you do this then the loop will be the length of your Logic session.