There is nothing particularly clever or advanced in this box. Each of the cab sections operated independently and is floating so as to allow for polarity reversal and complete disconnection from the rails when the switch is set to the off position.
I had decided to use a variable voltage regulator to control the output from each unit, and, once I had started to sketch out the circuit, quickly realised that what I intended to build was already available to buy cheaply. It then turned out that I actually had some of the requirements available in my component stock from a previous interest in robotics.
There are four of these small units in the controller. Two of them are simply used to set the maximum voltage while the other two supply the varying out put to the block control switches. The circuit diagram illustrating how to use the circuits are readily available on line. The chip that does the work is the LM317. If you want to know more, then ‘Google Images’ will give you lots of detail. I have decided not to give full details regarding the wiring of this unit as I have no control over how it would be used and mains power can kill.