We recently got the N&W J 746, which is the same train.  I noticed that every now and then it would stop on the tracks and not move.  It turns out one of the chrome rims on a front driver wheel has come loose from the rest of the wheel.  It then rubs up against and binds up the rods on the drivetrain.  I seems that the chrome ring has teeth on the inside of the ring which are supposed to bind it to the rest of the wheel when pressed on.  Unfortunately, this hasn't held.