Powering a Fastrack loop around a room ... implies this is a carpet central layout subject to disassembly and reconfiguration, so your power feeds need to be adaptable and easy to connect and disconnect.
Since you mentioned Powerpoles, you can get PP15 crimp blades that accept 16 to 18 Ga wires. Make a short pigtail for the Fastrack spade connection, or just use 16 Ga all the way the distribution point.
You didn't mention room size, but Ohm's law, amperage draw, and voltage drop over distance dictate the recommended wire size. 12 Ga is overkill for rooms in a typical home and you will probably find that 16 Ga is suitable for your needs. If you find the train slowing at the farthest points from the distribution point, upgrade the feed(s) to that track to 14 Ga.
