I have someone's older Bachmann HO passenger car set. I installed a sound decoder in his engine and he wants more light in his 3 passenger cars.
With the roof removed there is a single incandescent light in a well in the center of an acrylic light diffuser that runs the length of the roof. There is room in the well for a 3mm LED as well. The 3mm LED has a resistor and rectifier already on the leads.
Short of not having a keep alive is there any downside to running them both on DCC in parallel? It will be a simpler job than a complete LED conversion.