I wired my flatbed with 3mm LEDs. tailights are 5mm. The LED I use are a little more exspensive but come prewired with a resistor so there is no calculating required and a lot less soldering work. They also have switches and 9 volt battery boxes and a whole lot more. Great to deal with and good quality. I think I've had no more than 2 bad bulbs in the over 100 I've ordered.
http://www.modeltrainsoftware.com/led-small.html
Mario