However, many steam locomotives have metal bodies where one side of the headlight is grounded to one of the wheel sets picking up power. You may elect not to use the blue wire and leave the headlight grounded. It will operate at half brightness. I found this unsatisfactory. Trying to isolate the locomotive's original headlight bulb from ground may be impossible or nearly so.
Full brightness is easiest obtained with a grain of wheat bulb.
Replace Your Headlight With a Grain of Wheat Bulb. This is probably easier in a lot of cases than trying to isolate your head light bulb as described for the Big Boy below. It's also the way to go if you want full brightness, rather than half brightness. The grain of wheat bulb has no metalic contacts and therefore doesn't require any effort to isolate it from your engine's chassis. Also, you don't have to fiddle with the locomotive's electrical contacts for it's existing headlight.