Whats the best way to control voltage AC from 0 - 18VAC with a Arduino
A DC controller &quot;inside&quot; a bridge rectifier may work.
Feed AC via load bridge ACin and bridge AC out to AC2.
If you short Bridge + &amp; - outputs the load runs on full AC.
Now place a DC speed controller between bridge DC+ and DC-.
The controller (pot or PWM short or ...) sees rectified full cycle DC but load sees AC.
PWMing the DC load control (or using a variable resistor or electronically variable equivalent) will alter the AC



simulate this circuit &ndash; Schematic created using CircuitLab

It's likely that converting the whole system to DC would work - unless there is some  clever tricks hidden in there. eg points switching 'engines' rated for AC (higher inductance, with resistance too low for undropped DC). Unlikely, but ...
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