Hello, I just have an engine question.  I have two really nice engine cars one a Santa Fe (red and silver) the other a Union Pacific (yellow and gray) both are the long engine cars. They are the heavy duty long engines and both for some reason wont move. The light will come on and if i move the car wheels around a little the engine will come on and off but when i set it right on the track only the light comes on. Both engine cars do this the only idea I have is the power box im using to run the trains isn't strong enough? I use smaller engine cars (3 engines) and all work great but these two just wont run on the track. 

Any ideas? I'm just baffled the light comes on and stays on but engine wont run when set right on the track yet if i barely move the wheels or pick it up off the track just a little the engine does come on and off - Frustrated lol

Thanks so much,