The metal road wheels will add alot of weight, and my Hooben Elefant transmissions were pretty weak to begin with. I've had about 4 bent gear shafts (all on the lower end of the TU's). I replaced them with some spare Impact ones I had as extra. Now, problems are all gone.

As to metal tracks, I'm pretty sure Kenny Kong's are all that's available nowadays. They are the best for the Elefant though.