Puckdropper, using two LEDs in series is good. Use a 1/4 watt resistor as it has more surface area to keep its operating temperature down. Even though the LEDs are diodes, you should protect them from significant reverse voltage. use a bridge rectifier or a single diode in series (1/2 wave rectification from DCC track voltage) to further reduce resistor watts with almost no noticeable dimming of the LED.
If you are thinking about lighting passenger cars, you will need to add a Cary over capacitor to minimize blinking.
DonV
