There are several ways that DC can be dirty (not "pure").
The first and most obvious is DCC. It imparts a significant distortion on the pure DC that comes out of most DC power packs. This "distortion" is what carries the command information to the DCC decoder in the engine which then unleashes all the fancy bells and whistles. Makes sure that DCC is off or not connected to keep your DC pure.
The second is Pulse Power. This is designed to give better performance as the loco starts to move. Make sure that it's off. You may want to use it until the engine is rolling, but it needs to go off very soon afterwards.
The third is poor filtering of the power coming out of the rectifier inside the power pack. This rectifier changes AC coming in (from the wall outlet, and stepped down by a transformer) to the DC going out. Poor filtering means less purity. This is one very important place where you get what you pay for. Buy a good one. You won't regret it.
The MRC unit on the left in your picture looks to be a basic DC power pack with some of these advanced features that dirty up the DC coming out, like pulse power. Keep them shut off.
The other one may not even produce DC, although I could be wrong. It may be AC output; adequate for 3-Rail only.
The best way to tell what's coming out is with a tool that most of us don't have, an oscilloscope, which allows one to look at the quality of the output. Is is nice and flat, a straight line? Or does it have ripples? Are they tiny or huge?
Second choice would be a voltmeter, digital or old fashioned analog, either will work but you'll only really be able to tell whether you have AC or DC coming out and not how pure the DC is, if it's putting out DC.
Mike
