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