Search is your friend.

Search for "feeder wires" in the search box in the upper right of any forum page and you will find a long list of discussions related to feeders. 

This one is a good start:

https://forum.mrhmag.com/post/feeder-wire-gauge-recommendations-12187888

The Wiring for DCC is a great resource. (I am assuming you intend to use DCC)

Any wire capable of

Any wire capable of delivering robust voltage to the rails will do, so 18 gauge is fine.  I prefer thinner wire, say in the 22 gauge range.  Just keep them short, and use them frequently.  Once, I used longish 4' 22 gauge feeders at one end of a 6' bridge that afforded access to a five-track staging yard off the layout. There was no power provision for any of the staging; it all came from that distant pair of 4' feeders.  I felt there was no discernible problem.  My locos there all responded to inputs from the throttles.

I don't think you'd find many here enthusiastic about that setup, and I could only draw on this one instance for experience to refute them.  I might just have been on the very edge of suitability.  So, if you can keep your feeders short, that's a good rule.  Those 18 gauge wires should be good for at least 4' in length.

Once upon a time...

I once tested my layout with an entire 100 ft reel of 24 awg cat5 cable as a feeder. There was no observable difference between loco performance that way and with a direct connection via 18 awg. The voltage drop was not measurable on my Fluke meter.

I'm not saying do that, it was just a trial, but no one has ever reported problems due to short 24 or 22 awg feeders. The last inch of wire (in your loco) is 30 awg.

pqe

Wow, 18 seems huge for track

Wow, 18 seems huge for track feeders, let alone 16.  I used 22 on mine with no problems, plus it dissapears against the rails once painted.  I think everyone worrys way too much about voltage drop on layouts anyway.  Ive never sen a good technical argument for large track bus or large feeders anyway.

24 Gauge is Fine

My whole layout uses 24 awg feeders from cat5 cable.  Easy to solder to the rails.  No problems whatsoever.

18 is what I am using...

Yes, it is quite heavy for feeders in my opinion.  Lighter wire will be easier to work with and solder to rails.  It will also be easier to hide next to the rail.  I wanted to use solid wire and had it on hand.

The key to lighter gauge wire is as Pelsea mentioned, keep the runs shorter and it should be fine.

Regards,

DCC vs DC

The use telephone wire 22 gauge, I  think was common with DC. When I changed to Dynatrol they recommended a heavier 14 gauge, I think,and also with NCE for the Bus wire. As has already been said the length and spacing of drops is the main determining factor for size.

                                                                                                                           Ray

14/22

I'm using 14 gauge bus and 22 gauge feeders, ideally I want to have a set of feeders on every piece of track...IDEALLY!