Heat is mostly a non-issue for #18 wiring at the usual maximum current of ten amps, the issue really is the voltage drop at higher currents. 25 feet of #18 copper wire has a resistance of .16 ohms. At ten amps, the power dissipated by this amount of wire would be 16 watts. While that will probably warm the wire to the touch, it's not going to burst into flames. Also, it's VERY unlikely for you to be drawing anything like ten amps, especially running conventionally!
The real issue here is the voltage drop and it's effects on running. That would be my primary reason for recommending a larger gauge wire for the main power feeds. I used #14 wire for all my runs out to distribution points, then #18 wire for the short drops from the tracks.
