Banked tracks (where the outside is higher than the inside) occurs so that when the train (in this instance) is travelling at a certain speed, then there is no frictional force providing sideways thrust and hence reducing wear and tear on the wheels of the train and the rail itself. 
This occurs since the upward thrust of the road on the vehicle, acting perpendicularly from an imaginary line between the two rails, provides the required (horizontal) centripetal force to allow the train to round the curve. 
However, traction is still provided when the train is running along the line of the rail and direction of travel.