I was testing my 10th Anniversary Polar Express this weekend and tho I feared it would depart the rails at near full speed it did not. It still slows in the curves and I was somewhat dismayed at this, I expected cruise control for the price. Tho in looking back, it has the same electronics as much lower cost engines, just different sounds. I suspect the Movie company is making the price difference to use the name.
There is another way to slow the engine. Add pairs of diodes (wired back to back in parallel) in the wire to the motor. You get a 0.6VDC drop for each set. so 4 pairs will drop the motor by 2.4 VDC, 6 pairs will drop it 3.6V and so on. This lets you keep the lights in the cars on and not jump the rails at max speed. There are numerous threads about this on the forum if you need more details.
This also lets the kids twist the control farther to get less speed and get used to trains running at less than warp speed while your trains have the power to pull longer consists.
