If you notice that one wheel is wobbling sideways slightly when rotating, don't try and check it with a metal back-to-back gauge whilst it is still on the rolling road.
Because you will then remember
that even though the power is off at the controller, the track is still
live with 16V AC. And you will get a little crack and a spark when the
circuit is shorted by the introduction of said gauge...
Ho hum, live and learn, etc.