What is the purpose of a one-to-one transformer?

A one-to-one transformer serves a specific purpose in electrical systems, and the correct understanding lies in its function related to insulation. Specifically, option C highlights the role of a one-to-one transformer in insulating the third rail from the running rail. This insulation is crucial in transit systems, particularly in electric railways, where the third rail carries a high voltage that can be dangerous if directly connected to other conductive elements, like the running rail.

By providing this insulation, the transformer helps prevent electrical faults and enhances safety by segregating the high-voltage components from other parts of the system, thus protecting both the infrastructure and the personnel working nearby.

The other options refer to general transformer functions rather than the specific application of a one-to-one transformer. Step-up and step-down functions relate to how transformers can change voltage levels, but a one-to-one transformer maintains the same voltage level while providing isolation. Controlling current flow pertains to devices designed for that specific purpose, not the function of a one-to-one transformer. This targeted purpose makes the correct choice particularly relevant in the context of transit electrical systems.