As a sibling comment mentioned, it's possible there was a diode somewhere in the system. Another possibility is that the motors in the slot cars had electromag

As a sibling comment mentioned, it's possible there was a diode somewhere in the system. Another possibility is that the motors in the slot cars had electromagnets for the stationary field instead of permanent magnets. That's what an ordinary power drill, especially one without electronic speed control, has. Such a motor will run pretty much the same on AC or DC. When running on AC, the rotor and stator both change their magnetic fields in perfect sync so the net effect is substantially the same as running to same motor on DC. Wikipedia has an article on "Brushed DC electric motor" with a section on "wound stator". It doesn't specifically mention running the motor on AC, but there's enough there to show you what's going on.