How can I build a diamond shaped minecart track?

You can make a diamond shape that you can go around continuously by holding one direction, but it won't have smooth edges. The wiki page's mention of bumpers and intersections leads me to believe it should look something like this style of Skeletron Arena, but smaller:

The extreme left and right ends of the track have been hammered to be "bumper" endings that reverse the direction of the minecart, while also reversing any currently held movement key so the player can hold a single direction the whole time. Letting go of your movement key will restore your usual controls. The two intersections near the the bumpers have been hammered to direct the minecart around in a loop. Besides those, everything is normally placed track.

Starting anywhere in the diamond and holding either direction will soon result in moving counter-clockwise around the diamond. One could easily hammer the intersections differently to go around clockwise instead.

And there's rail at the top that I couldn't fit on screen, of course. That rail in the bottom left was just a way to get onto the diamond, and can be ignored for Torch God purposes.