Even before I got my new Emerald Night I was thinking about running it off of my (classic, electrified) 9v tracks. Here is how I did it...
I took two brushes from a RC car set.
I cut a power functions extension wire in half.
The ends of PF wire are connected to the brushes.
The tender is slightly modified with a weight, to keep the brushes down; and a light, so I know when the train has power.
The Emerald Night has the PF XL motor, lights, and IR receiver in it. And I use the IR Speed remote.
Here are some pictures of my conversion:
http://members.shaw.ca/mi_spam_account/IMG_2653%20small.jpg
http://members.shaw.ca/mi_spam_account/IMG_2654%20small.jpg
http://members.shaw.ca/mi_spam_account/IMG_2655%20small.jpg
http://members.shaw.ca/mi_spam_account/IMG_2656%20small.jpg
It doesn't work perfectly. Dead spots, like switching tracks cause it to stop if I'm not going fast enough. I tried the lego capacitor, but it didn't have enough voltage. I really need a 9v capacitor.
I also found the train doesn't like changes in elevation.
I did this because I thought the PF rechargeable battery box was too expensive. It is almost the best of both worlds - IR remote and electrified tracks. I just need to get it working more smoothly now.