I've done this before, but not to the Polar Express observation car. Adding exterior lights, that is. When Lionel first released their "new" heavyweight cars, I found that grain-of-sand bulbs will fit inside the marker lights, almost as if Lionel planned on it. It involved making a constant voltage circuit for the 1.5V bulbs, but that presented little problem. For the PE car, I contacted Even Designs, and they had AC/DC 7V-19V lights listed that worked out even better. I ordered their "pico" sized ones that are unbelievably small, and fit inside the light housings just fine. The "warm white" ones come pre-wired with the appropriate circuitry so they can be spliced into the bulb socket in the car with minimal work and they shine light in almost all directions. For the upper tail light, their AC/DC 7V-19V red, 3mm light worked as easily once the factory lens was removed and the hole drilled out. Here's the installation with the car on the tracks and 15V applied:
A view in a darkened room:
Going into a tunnel on my unfinished layout:
A close-up of a marker light:
Marker and tail light:
The actual color is more red than shows in the photos, and the contrast isn't as stark.
This little project cost about $10 total and was lots more fun than raking leaves. And WAY more fun than shoveling sn*w (a four-letter word) 
