Wow, just read most of the site you gave the link for, good stuff there. It reminded me of school, voltage, current, resistance,if I never have to figure out another combination circuit again, it will be too soon. All his work points to the fact that there is no electrical reason to go larger than #12 unless you have a staduim sized layout.
Current x Voltage = Power
14 Amps x 110 Volts ~= 1540 Watts for microwave oven
0.5 Amp x 4 Volts ~= 2 Watts for HO peddler freight
4 Amps x 6 Volts ~= 24 Watts for heavy drag freight
25 Amps x 10 Volts ~= 250 Watts for really big model railroad
Current ² x resistance = Power Loss
4 Amps ² x .0045 ohm/ft x 40' x 2 = 6 Watts power lost in long #18 wires
4 Amps ² x .075 ohm/ft x 10' x 2 = 24 Watts power lost in long code 70 rails
It is the second equation that can cause the problem for model railroads.
The high current through the slightly resistive wires and rail causes small voltage (power) losses so the trains don't see all the voltage or power. If the out and return wires are long, a loss of, say, 1.5 volts, then this may be very noticeable to the train, but not the microwave oven.
I operated on a friend's DCC layout and there were three places where trains would suddenly speed or slow. There were auto light bulbs (for current short protection) in the loop as well as the long wires, so part of the problem may have been differing bulbs. The builder was careful to use heavy wire and many feeder wires, but the different power blocks still were noticeable.
Another possible cause may have been some smaller wiring in the power routing circuitry for a group of switches.
Sorry, Internet Explorer suddenly blinked out and flashed and dissappeared. I had not even hit post yet. Then there were three identical posts.