Each carriage has its own dynamo which is belt driven from a pulley on one of the bogie axles. Here the dynamo will be driven in a similar manner by a variable speed motor, mounted beside the rig, controlled by the two red handwheels, operating the mechanism at the base of the test rig.
To control the dynamo voltage at just above the nominal 24 volts a regulator is required. This is the large black box that can be seen at the top of picture above. The regulator consists of a collection of coils, wire resistors and switches to achieve the voltage regulation and retain it at a steady level at all speeds above 25 miles per hour. Each carriage is fitted with a regulator.
When the dynamo is at rest the voltage required for the lamps is supplied by a bank of lead acid batteries which can just be seen at the rear of the test rig at the bottom right.