FWIW attached is a snip from a wire loss spreadsheet I cobbled together a few years ago. At a glance you can compare side-by-side permissible wire run lengths for common wire sizes for various common amperage loads, based on the common out and back scenario, and 2 volts max loss, which is a good value for train wiring. The max suggested load current for each wire size is shown along with the max recommended fuse or CB rating for each size. This sheet has made life a lot easier for me.
Also for more versatility, attached is the fully functional Excel spreadsheet. It has only two inputs: you can select either out and back or one way wiring, and you can select the allowable voltage loss you are comfortable with. Once these selections are made the allowable wire run lengths are all 
recalculated accordingly. I am putting this out there for anyone to use as they see fit and I make no guarantees as to it's accuracy or suitability for any purpose. 
The spreadsheet is not protected in any way, so use it accordingly. Caveat emptor.
Rod
