Try raising the voltage. The silver wheel is most likely the magnetic sensor used for activating early railsounds rev up.
20 volts wouldn't do it. But I did find that the solenoid, spring & plunger was just a bad match - the coil doesn't pull the plunger away from the knuckle quiiiiite far enough to rekease it. After playing with it a while, I clipped the spring, about 1/3 of a turn at a time - tried it, and if it didn't work, I pulled it out and clipped another 1/3 turn. It took somewhere around 1.5 to 2 turns clipped the spring so that the plunger was pulled back far enough to release the knuckle. Works like a charm now - just finished modifying the third one and they all work great!
You are right, the little flywheel doohickey is a magnet. Totally unclear to me how that could trigger a pulsed signal to send to a sound board????
Any idea on where this truck was used and/or what it's part number might be?