There are many folks who might disagree with this, but 12 gauge stranded is said to be the best way to go. The stuff made by Malibu for their ourdoor lighting. 70 feet of their 12 gauge copper stranded double wire can be buried wire goes for $70 dollars on fleabay. That is a lot more than I paid for it 10 years ago, but then copper has more than doubled in those ten years, too.
I’ve been told why stranded is better than solid wire for our application, but that knowledge has dribbled out of my flea bitten former military mind. It made sense at the time, though.
Come to think of it, it might be less expensive to use the Hillmans or Split Jaw to achieve a good mechanical and electrical bond. It depends on how many track joints that you have. If you are using 60 inch flex track, you will only need a few. If, like Ralph, you are using 1 foot lengths of track because you were given a truckload of it, then you might want to go another way.
I don’t think that 18 gauge wire will support a large scale layout. It is not even recommended for a 0 gauge layout. 14 gauge is the smallest that is recommended to 0 gauge. 18 gauge might work for H0, I don’t know. I think that you will be disappointed if you try to use the 18 gauge.
Now, as to how to connect the wires to the track… Some folks solder the copper wire to the underside of the track. That certainly gives a good mechanical and electrical bond, but if the track is already down, that is probably a non starter. Hillmans, and probably Split Jaw, makes a clamp that you can use to attach the wire to the track. So does (did?) LGB. I’ve used all three. Here is an example of the Hillman. As you can see, if you use Hillman or Split Jaw you could attach the wire directly to the regular track joiner.
You can try soldering the wire to the Aristo track joiners. That might work. Hillman even has over the joiner rail clamps.
Or, you could look into R/C with Battery control… Did I say that out loud?
Good luck.