I remember the Tomar Constant intensity lighting kits from my HO days in the 80s. I did a little web searching, but every link to the circuit is a dead link (where old technology goes to die . . .)
Anyway I sketched the circuit here (ignore the diode part numbers) :
The idea is that each diode drops 0.7v, so you get a constant 1.4 volts differential across the pair, enough to light the 1.5v incandescent bulb in the loco headlight (or up to 4 bulbs according to Tomar in parallel, eg. in a passenger car). This was designed for variable DC track voltage. The 1.5V lights fully as soon as you apply transformer power (transformers usually jump from zero to a few volts), then the 14V ballast bulb gradually gets brighter as you increase the voltage. The reason for the 2 sets of diodes was for the polarity change when you changed loco direction.
It was a pretty simple solution but it did create 2 problems: Light and heat. That is the reason for the silver tape (and the suggestion to add a 2nd 14v bulb in series so each bulb burned cooler). I seem to remember people occasionally complaining about melted HO loco shells.
Since this was posted on the 2-Rail forum, I would assume that this is a conventional DC locomotive, thus the use of the same technology as DC HO.
GRJ's constant intensity modules are a vast improvement, don't you think? 
Bob
