I'll just add to Steve's reply by saying that as a DCS user I only segment the power rail, all commons are tied together. Careful, common and ground are not necessarily the same.
When you think about an engine passing through a segment (block) the majority of the current is flowing to the rails on the one feed to the block and yet there is an entire common rail with several drops that provide more that adequate return path to the transformer. I am assuming all drops and feeds are done in pairs.
I cheated and use whatever I could find in my box of cut up wires. All of my feeders are 12 gauge stranded wire, some only a few feet long. I have a few 18 gauge drops but most are 16, may be one or two 14 ga. drops.
