It is all of the turnouts. 

         I recently purchased Thomas with sound (Item 58701).  It runs well and the sounds are great. BUT, it tends to derail when passing over a turnout, whether it is going straight through or turning off.  I have many engines and none of them have this problem.
        The Thomas engine has a protrusion on the underside, a gear box.  This sticks down a lot, almost to track level. I suspect that this this part is striking the frog and causing the derailment. Thomas gets onto the turnout ok, but derails when it is half way through.                   
        I also bought a Henry the Green Engine. It has more wheels but nothing sticking out of the underside and it has no trouble passing over turnouts.  My track and turnouts are quite new, as I installed all new track, turnouts, and wiring. As I am past the 30 days, I cannot get a refund. Yes, I should have checked it out sooner, but waited till some of my grandchildren were coming before testing it. As it is new, I did not foresee any problems.  Would any of you have suggestions to fix this?