If you are using block wiring, you don't need to isolate the outside (neutral) rails, unless you are using a track section as a detector section, which you likely are not doing.
When you say track feeder, are you running the 18 gauge from the control panel/block switch to the block/track involved?
Normally from the block switch or transformer, I would use a heavier gauge wire (like 16), then you run the feed wires that solder to the track off of it. If your layout is small and you have only 1 feed wire to each block, then using 18 would likely work, but personally I would bring the power to the block using a larger gauge and then run the 18 gauge feeder(s) off of that rather than use 18 all the way. It sounds like you have a small enough layout the 18 gauge approach may be doable. I don't think you will have trouble with heat using 18 gauge unless you have something drawing a lot of current.
