@Squirrelstrains, You would never feed more than 18VAC to modern O Gauge Equipment. Therefore your premise of feeding 20VAC is invalid and should not be done. That is because the peak voltage, even if throttled, is still 20VAC (see the chopped AC waveform above).
AC meters are used to measure the RMS value of sine waves. True-RMS meters are required to measure the RMS value of complex wave forms, such as chopped sine waves generated by a triac. Non-true RMS meters actually measure the DC voltage and multiply it by .707 (appropriate for the pure sine wave) to get the AC voltage. Think of RMS as related to the measure of area between 0 and the wave. The more pronounced the chop, the less area and therefore less apparent RMS voltage.