This PWM (pulse width modulation) controller is a ready built module. Simply connect your 12v DC and your track. (Easy screw connections). If you want to reverse direction (could be useful) you will need to install a suitable switch. One is listed by this seller. The module comes with a proper knob. The knob has an overall diameter of about 29mm. Module can take up to 20v DC, and 2 amps continuous. Rapidly despatched from London.
eBay