Lee,
I'll try to make this simple.
What you basically need to know is this; what is the power output of your power pack in terms of amperage.
Most N scale engines draw .75 amp of power, HO will draw 1.0 amp, On30 will draw 1.25 to 1.75 and G can draw 2+ amps. These figures are approximate and are for one engine only.
How to determine the output of your power pack is easy. On the back there is usually a rating like 17VA. That means 17 volt/amps. So what is that? You are operating at 12 volts, so divide the 17 VA by 12 and you get a power rating of approximately 1.41 amps. The MRC Control Master 20 units I used are rated at 85VA or 7.08 amps.
Now you take this figure and divide it by the average loco draw from above and you will get the theoretical number of locos you can run with that power pack. Phew!
But wait! Now you have to deduct all those interior car lights. They also draw from the power available.
As to your question about what to get to run G scale, get a pack designed for G. The MRC CM20 will work as well as other specifically designed power packs for G that have a higher VA output. Today, heat is not as much a problem since all the newer packs are printed circuit and not wound resistor throttles.
What MRC units are you using now?
For the answer to the face/reverse running; a DC motor has a plus side and a negative side. The industry standard for model DC locos is that the "right hand or engineer side of the rail is positive" and the left is negative. A DC motor does not know this and thinks that which ever side of the motor or loco is on the right side is getting the positive feed.
So, one engine facing forward will get the positive feed from the right and the other, the reverse loco, will get the positive feed from the right, but it thinks it is getting that feed from the left. So one motor in this sense will run clockwise (ie: forward) and the other will run counter-clockwise (ie: reverse) and as a result both will run in the same direction. Hope this helps.
Long winded again.
B-)
