I should say the bipolar current waveform that although is rectangular is NOT in any time sync with the DCC voltage waveform. The current pulses come from decoders that are actively driving motors with pulses (PWM) asyncrounsly with the DCC voltage waveform. it is a random composite of all the decoder motor current pulses that are running. But the end result through the rectifier is still DC although pulsed DC….the meter will report the AVERAGE value of the collective current pulses. It will NOT report the peak current which is what the booster monitors.
The complexity of the voltage waveform is the same for the current waveform. Both are bipolar pulses. A waveform that the most common DVM meters are not designed to read correctly. The bridge rectifier removes the bi-polar nature of the rectangular waveforms to a unipolar output. Given the waveform is a rectangular, the end result is that the rectifier converts the rectangular AC to 99% DC which any meter can measure accurately.
As Don said, there is calibration issue when reading voltage. You ave to account for the two diode voltage drops.
For current, the bridge rectifier does not introduce an error term but does reduce the final track voltage by two diode voltage drops. If a locomotive crosses a booster district where the track current is NOT monitored to one that is monitored, you can see the engine speed change. However, higher end DCC decoder with BEMF motor drive will minimize/compensate for that effect.
Taking a step back, I am always wondering why people think is it important to monitor the booster voltage and current. If you think it will give you a heads up on booster overload, that is not what actually happens. Most overload problem are not related to train running issue. They are recovery from a short circuit while dealing with high inrush current issues.
Well, I would except that the space available is barely larger than a single RRamp circuit board. I guess I don't really understand the issue of correctly measuring booster output -- I would speculate (and please correct me) that current flow should be straight forward, but voltage measurement for the DCC waveform is probably 'interesting'. I have a cased RRamp that I use for spot checking the track, so I probably don't really need running voltage from the booster or the idiot lights. If it ain't DCC coming from the booster than I probably have other issues.
Anyhow, if a regular digital amp meter will provide a reasonable value then I am probably ok with that.