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Some old MRC power packs do not provide smooth DC, and we have one at the club that is not turning off with the power knob turned down to its stop. It is putting 0.2 VDC and 2.3 VAC on the rails when turned to zero speed. (For Peteski - yes, that was measured with the Fluke 36, not the cheap multimeters that were giving strange AC readings with DC switching power supplies and even batteries.) We first noticed that because it was causing a loco to buzz when it was stopped at zero speed on the MRC controller knob.
...You can change C1 and C2 from .01uF to 1/10th of that, or .001uF,...If you go down to 150 pF, you can get around 15 kHz, which should work pretty well and get rid of your buzz......It works by raising the ground reference up and down... When both rails are at 12v, the train will be stopped because there is zero differential between them... But it might cause unexpected problems one day if you ever connect any other electronics to the rails that assumes that both rails are at 0 when the train is stopped.
It really doesn't matter which rail is called "common". What is odd here is that both rails will have a positive voltage on them relativeto ground. Consider the case where the train is stopped. Both rails are at +12 relative to ground. If you were to connect a lamp between either rail and ground, it would light up. With a "normal" DC controller, both rails would be at ground and any load connected between either rail and ground would not light.
Other options ...would be to change the pot and resistors, so you can use higher values for C1 and C2. There are lots of value combinations you could use to change the R/C timing to get a better frequency with stable and commonly available values of parts. I'll play with it more when I get it...