A VARIATION: I made another "Children's Layout" for the Exploration Discovery Center in Grover Beach, CA. While the layouts I built have a means to limit maximum speed by limiting track voltage, they wanted slower speeds than possible by that means. They made their own top bars for the cradle to further limit maximum speed by further limiting throttle rotation. I have no drawing, but you can see the difference in the side-by-side photo with the original on the left and modified on the right.
With the top bar they made, the throttle knob protrusion hits the edges created by a round feature cut into the top center of the bar. I'm a little concerned that it might act as a fulcrum, and it might be better to have a design where the tip of the protrusion hit an edge. But it has been working. Also, they could have made the recess on the bottom edge a little wider to expose more of the bell and horn buttons, but this is minor.
The original bar is not meant to limit speed, although it does somewhat. It is meant mainly to protect the internal stops of the controller's potentiometer from breaking. Controllers have now been working for three years with heavy use on the layout I built for a railroad museum and there has been no damage to the controllers.
The two photos below show the two layouts using these "Controller Cradles."
