In another post I referred to a possible solution to the under powered wall wart power packs supplied with the Polar Express sets.
I bought a referenced power supply and here are the results of the adaptation to the PE set.
First I purchased the power supply for $12.99 with free shipping:
I already had a 5.5mm OD X 2.5mm ID plug from another project. 
The included power cord can be inserted into the power pack either way...it won't effect voltage output or polarity.
I tested the pack with a volt meter and verified that it put out 18VDC.
The next thing to do was to strip the jacketing off the wire near the 4 prong plug to see how many wires were inside. There were only two wires...one brown and one blue:
Next I cut off the 4 prong plug and discarded it as it isn't needed:
I stripped the brown and blue wires to check the polarity of each:
The brown wire is the (+) positive wire. The blue wire is the (-) negative wire:
The pack puts out 18VDC: I used the striped wire from the plug (goes to center hole) to mate with the brown (+) wire
Now solder the plug wires to the pack wires and heat shrink each wire separately AND THEN BOTH WIRES TOGETHER. DON'T FORGET TO SLIP THE HEAT SHRINK TUBING OVER THE WIRES FURTHER DOWN THE LINE BEFORE BEFORE SOLDERING!
Test the pack again with a meter:
Now it's time to test the pack with the track.
I assembled the loop of Fastrack supplied with the set. Assemble the sections carefully and run your fingers over all the rail joints. If you feel two adjoining rails are uneven disconnect those two track sections and try again. I had 3 joints improperly connected on my first try.
Now plug the power pack into the straight track power section, plug in the power pack to 120VAC and see if the red power light comes on:
With the (+) brown wire from the pack connected to the striped wire on the plug if you check the track polarity you will see it reads the outer rail is the (+) rail and the inner rail is the (-) rail. A meter shows the rail has 18.10 VDC supplied to it:
Turn off all power, put the engine on the track, turn power back on and turn on the remote.
The engine will start up and run properly:
I then tested the engine with all five passenger cars to see if the new pack would operate the train. IT DOES!
You now have an 18VDC 5.0 AMP 90 Watt power supply to use with your new Polar Express set!
The best part is even if you have to buy the new plug you'll only pay $15.80 total for the new power pack and plug.
That sure beats Lionel's $55.00 plus shipping for the Lionel 6-81603 that only puts out 72 watts.
As before in the other post I will now add this disclaimer:
Disclaimer...You use these modifications at your own risk (as with all things in life) and I make no representations as to the operability or compatibility of these suggested changes with your equipment. I make no warranty expressed or implied as to the successful operation of these modifications with your PE set.
That being said, I'm very happy with my new power pack and won't have to worry anymore about my engine starting and stopping due to insufficient voltage or wattage.
Good Luck!
Mark
PS This power pack will most likely also work with O gauge fastrack Lion Chief and Lion Chief + sets.
I haven't tested my pack with an O gauge set as I haven't a loop of O Fastrack with a straight power section handy ,but the 5.5mm OD X 2.5mm ID plug is what is used on the 6-81603 pack. As long as the (+) side of the 18VDC powers the center rail you should be good to go.
(See disclaimer above!)
PPS You probably noticed some of the passenger car lights flickering or completely out. I didn't clean the fastrack loop before operation nor the car wheels. Rest assured that the lights do work. I'll probably swap them out for LED strip lighting sometime in the future and that will further decrease the power needs of the set as the strip lights use very little wattage when compared to the incandescent bulbs in the cars.
