Here’s a simple circuit for Passenger Car (or even Caboose) interior lighting. We use the 12V bulb as a “ballast;” the ballast absorbs the voltage over and above what’s required by the 1.5V bulb(s). The 1.5V bulbs begin to glow with with less than 4 volts on the track, put out good light with 5 volts, and maintain relatively constant brightness up to 14 volts on the track. Since there’s nothing polarity-sensitive here, the lighting is on for both forward and reverse directions. The diode bridge is connected to provide 2 series diodes in each direction, limiting the voltage which can develop across the bulbs to approximately 1.4 volts, thereby prolonging the life of these delicate little critters; almost any bridge will work, so shop price.
For those of you enjoying DCC on their layouts (like me), this circuit works great! But…IF you run greater than 14 volts RMS on the track, you’ll probably want to replace the suggested 12 volt ballast lamp with a 14 volt beastie, such as Miniatronics 18-014. If the illumination bulb aren’t as bright as you’d like, try using two of the 14 volt ballast lamps in parallel to ensure plenty of current for the 1.5 volt bulbs.
Below is a circuit which is similar to the previous one, in that we’re still using the “ballast lamp” — only this time we’re using LEDs to provide the illumination. I like this one best, as the LEDs begin to glow with only 2.5 volts on the track, and maintain full brightness from 5 to 14 volts. Note that no diode bridge is needed here (the LEDs are their own diodes). Yellow LEDs are much cheaper than white ones, and the yellow light is acceptable to me; if you insist on a whiter light, be prepared to pay more for the relatively new white LEDs; I haven’t tried blue LEDs, but they’re another possibility. This circuit is best for DC cab layouts (although it can be used with DCC); for DCC layouts, try the circuit just a bit further down the page.
For the analyticals amongst us, the graph below shows the voltage across the bulbs or LEDs with respect to the DC track voltage for both the circuits above. Note how quickly the LEDs peak and flatten, indicating “earlier” and more uniform illumination in the car.