I have replaced the wheels today with the ones with tyres on and what a difference,I was a little concerned that over time the motor's might bun out as the 4 car set seemed to struggle especially on curves and a very slight incline.
If anyone is interested in doing the same and you are not too sure what to do have a look at Budgie's post 446 above,and follow that,it took me about 5 mins to change both cars,no need to dismantle the whole unit as recommended by Bachmann when `I spoke to their technical dept.