First, What John said. Simple, the more power feeds you have the less voltage drop you will see around your layout. I use 14 gauge wire from my transformer to my power distribution blocks. From the distribution blocks to the track I use 16 gauge wire with connections about every 5 feet or so. I also don't rely on a switch or cross over to feed power to an adjoining section.
I have a PW ZW that I use for my primary power, which is limited to around 18v. And my oldest PW engine runs a fairly consistent speed around the entire layout. I also have an early TMCC engine with 3 poll motors that runs perfectly with this setup.
