I got a L'il Big Haulers set for my son about 10 months ago and it worked fine until about a month ago.  Now the engine runs at less than half the speed and has no power when the cars are hooked up. 

I first cleaned the engine wheels and track with a soft cloth and there appears to be minimal crud on both.  Next, I checked the voltage on the track and it varies with the controller setting as would be expected and is about 16V when at the maximum setting. I also took the terminal and applied it directly to the engine wheels and the wheels ran at about the same speed as when on the track.  So there does not appear to be anything wrong with the controller or the track.   

The engine is less than a year old, so it is still under warranty, but before returning it to Bachmann, it would be nice to confirm that this is really a problem with the engine.  Any other thoughts about what this problem is?

thanks in advance.