Just a quick intro . . . One of the idiot-proof things that I love about lego monorails (at least 6990 & 6991) is that they have lights so that you remember that have the battery pack on. It's on the same, but my PF trains all have lights in them for the same reason . . . and it looks cool.
At first I was stupid enough to buy the parts so allow for a light on the roof-top in place of one of the vents, then came up with something a little better.
The lego 6399 is the only monorail train that isn't equipped with a 9V light, so . . . . someone's probably done this about a billion times, but I've tried to make a mod to the train to get it lit up on the front end. It extends the length of the train car 2 studs and there's a one plate height increase. I've pictured it with the prior to updated rear car. Would love to hear opinions. I do wish that the front windows would line up perfectly, but I think that it's an improvement in appearance. With a proper length cord, there will be no loss in seating. The second car, would have a one seat increase in capacity potentially.
Just a few pics.