It's about 18 gauge but it's only about a foot long. Amperage depends on gauge but also on the length of the run.
I would wire the 18 gauge a foot or two to under the layout and then buss the power with at least #14 around the table and pick up the #18 wires from a number of points on a track oval for example...I have only 2 #18 terminal sections on 12 by 14 foot ovals which seems ok. I did have a slowdown on one corner and I just inserted another terminal track there and wired it's #18 to the buss.