This post is on my second hardest conversion, my observation car. It originally had seven 3mm 1½ volt 20 ma GOW bulbs. Three individually lighted rooms, four bulbs in the lounge/window area and three micro bulbs 1mm 1½ volt rear "taillights".
The original 10 bulbs current draw was 200 ma at 1½ volts, by tweaking the LEDs I was able to light every thing using four LEDs. All four LEDs are 3 mm warm white and inserted into Evergreen #226 (3/16" OD) Styrene tubing, the three room LEDs have 1K Ω ⅛ watt resistors in series and draw 1.4 ma at 4 volts. The fourth LED has a pair of paralleled 100 Ω ⅛ watt resistor to make a 50 Ω resistor and it draws 17 ma at 4 volts. The fourth LED drives the rear "taillights" as well as lighting up the lounge area. The rear lights use .040" optical fiber strands with red glass stain on the ends. The Styrene tubing acts as a defuser for the LEDs for better light dispersion in the seating area and rooms. With the high intensity of the rear LED I used a .020" thick Styrene sheet to make a ceiling over the lounge area to reduce the light from the LED.
The total current dropped from 200 ma at 1½ volts to 21.2 ma at 4 volts.