What you are experiencing is normal for DC operation. As the loco experiences more friction on the rails as it travels around, it requires more power to overcome the friction. Hence the voltage drops and the amperage increases. This is part of Ohm's law which says that voltage equals the amperage times the resistance (V = I*R). The equation must always remain balanced. So, as the resistance increases, the amperage decreases and the voltage remains constant. Or, to put it another way, as the resitance increases, the voltage increases and the amperage remains the same. In most cases, it's a combination of the two where the amperage increases, the voltage increases.
Since you have no control over voltage and amperage, (unless you want to buy a different type of power pack - and that can get expensive) you have to take a look at the resistance. While you have no control over the electrical resistance, you can control some of the physical resistance.
You mention that you are using an Athearn AC4400. This is a 6-axle loco. What is the radius of your circle/curves around the tree? If they are less than, say 36", there will be physical resistance which will result in electrical resistance as these long trucks try to negotiate tight curves. Can you increase the radius of the curves?
You mention that you have an over-and-under layout. How steep are these grades? What is the maximum height of the over-and-under? What length of track does it get to the top of the grade? I assume that as the loco climbs, it also goes around a curve. These are all elements of physical resistance (and hence electrical resistance) as the loco tries to climb the grade. Is there any way you can lengthen the track as it goes up the grade? Can you reduce the height of the over-and-under?
Have you lubricated the bearings, the drive train and the motor? Follow the manufacturer's instructions in lubricating.
How new is your loco? If it is new, it has a high physical resistance in the motor, the drive trains, the bearings, the wheels, and hence a high electrical resistance. Run it around the track with no load for a couple of hours. Then change it end for end and run it around the track with no load in the other direction for another couple of hours. This will break in the loco. And don't forget to lubricate before breaking it in.
Finally, given that it is an Athearn, these locos draw a larger current than, say, a higher priced Atlas or Kato locomotive. And, as it is a 6-axle compared to a 4-axle, it will also draw more current.
As long as you've reduced the physical resistance to the extent possible, you've lubricated the loco, you've run it in, and the voltage and amperage is within the parameters of your power pack, sit back and enjoy your train!
Or, better yet, go down to your local hobby shop, pick up a couple of switches, and some track, and convert that over-and-under circle of track into a layout with some sidings.
Bob M.