Does the red light turn on only when there is an overload?

On a postwar LIONEL ZW TRANSFORMER, Yes the red light will come on if a short is present with the throttle on. As soon as the throttle lever is in the off position the red light will go off. Shortly followed by the automatic circuit breaker clicking, as it cools and resets. The short on the track may still be present but with the lever in the off position the red light will stay off. There are several variations of the ZW 275 Watt transformer, early and late "type R" models. Both work the same but are wired slightly different the early ZW uses two 6 volt bulbs and the "Type R" uses two 18 volt bulbs. Internally if there is a resister connected to the base of the red light it will use the 6 volt bulbs.