Welcome to the forum!
To properly diagnose the problem, a few steps need to be taken.
  Is the transformer powering only the North Pole Central train, or other trains and/or accessories also?

  Make sure all locomotive wheels and pickup rollers are clean, as well as the track. Clean with hardware store naphtha if necessary. Make sure everything is clean before proceeding.

  If you have a voltmeter, measure the AC volts at the transformer terminals, and also at various points around the track when you first power up the trains.
Run the trains until the slowdown occurs, and do not move the throttle when the slowdown begins. Measure the voltages again. If you get voltage readings that are within a volt or so of the original readings, chances are the transformer is good, and the locomotive may have an issue.

If you do not have a voltmeter, observe the locomotive headlamp, or any other lighted car to see if the brightness changes before and after the train runs for a while and slows down.

If you have another train or transformer, you can swap out one or the other to help diagnose the cause.

Please report back if you still have any issues after performing the above steps.

Larry

The North Pole Central set is a fairly modern set, from 2009, and the locomotive has a DC 'can' motor and an electronic e-unit.

The servicing information referenced pertains to the older AC/DC universal motors and the legacy electro-mechanical e-units.

The DC motors of today, as well as electronic boards, are not serviceable for the most part, and just get replaced if found to be faulty.

If the track, wheels and pickup rollers are clean, and the transformer passes the voltage test, then the likely problem is either the DC motor, or the electronic e-unit board.

You may wish to put a drop of oil on the locomotive axle bearings and a dab of grease on the gears to make sure the locomotive is lubricated properly.

You may also wish to run just the locomotive and tender alone to eliminate any of your cars from causing a problem.

Larry

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Glad you found the problem.

Compliments on your multi-gauge layouts. You have packed a lot into a small space.

Merry Christmas!

Larry