Here is a little something I whipped up to make visualizing allowable wire lengths and AWG gauge sizes a little easier. It uses Ohm's law and published wire resistance values. The default allowable voltage loss is 2 volts; ie if your transformer is set at 18 VAC, the actual differential voltage getting to the track (or whatever) is 16 volts. There is a 1 volt feed wire loss and a 1 volt return wire loss; total 2 volts. You can select whatever voltage loss value you like and see the effects on the wire lengths you can use. The lengths are based on out & back runs. In other words a 50 foot run from the source to the load is actually 100 ft of total wire length.
You can open this .xlsx spreadsheet with any version of MS Excel from 2007 on. You should also be able to open it with Apache Open Office, though I have not tried it. Anyone who cannot open it shoot me an email and I'll send you a pdf version that you can open with Adobe. 
Have fun! (Note it's an unprotected worksheet!)
Rod
