Bought a Bachmann used controller model 46005a on EBay for $17.50 and it works perfectly. Problem solved. Thanks for your advice JBrock and Richly.

Yes. It's the same controller.

I checked the controller knob like you suggested and looked at it with the back opened but there is no way to tell if the shaft is broken. However, the knob is probably not working correctly.  I think my grandson was changing the direction switch without turning it off first. Anyway, I am bidding on a used controller on E Bay. thanks for your response, Rich.

the controller only runs at full speed when turned on. It no longer changes the speed. Is there any repair that can be done?