Here, I have used a computer's power supply to power the whole setup but you can also use a 12-volt power adapter.

You can also test your setup by just using the locomotive.

After powering on the setup, press the button on the servo tester once. The middle LED should light up and the servo tester will command the servo driver to move the motor at 90 degrees angle(As it would normally do in a regular servo motor). 

Adjust the potentiometer on the servo driver until the locomotive stops moving.

Since this model train driver uses a servo motor's driver board, it can easily be controlled using any microcontroller and can also be used with Wireless RC controllers to control the trains and even turnouts. Excited? Stay tuned for future projects related to this and all the best for your build!