Frictionless banked turn, not sliding down an incline? I was thinking about the problem in terms of defining the axes as follows in this answer, where the author states that:
$$mg \sin\theta = (\text{Centripetal Force})\times \cos\theta$$
However, I don't see how the forces acting along the incline end up cancelling in this situation. Both $mg \sin\theta$ and $F_N(\sin\theta)(\cos\theta)$ are working in the same direction. They are both working to bring the car down the incline. So shouldn't the car thus be sliding down the incline because both a component of the force of gravity down the incline and a component of the centripetal force are bringing the object down the incline? I know that this is not the standard way to think about it as defining the axes as such nontraditionally. However, I want to see how it works even despite this.
This answer begins to address my misunderstanding, however it uses the idea of centrifugal force which I come to understand as fictitious. Thank you.
 A: 
The vehicle is moving in a horizontal circle with a constant speed. That means it is constantly accelerating towards the centre of this circle. (Acceleration does not have to be a change of speed; it can be a change of direction, or both.) The acceleration is $a=v^2/r$. 
Newton's 2nd Law $F=ma$ applies here; $F$ is the net force on the vehicle. There are not 2 horizontal forces acting on the vehicle (centripetal force, component of normal reaction). There is only 1 force (component of normal reaction, which is $N\sin\theta$). This force is the centripetal force. $F=ma$ tells you how this force $N\sin\theta$ is related to the centripetal acceleration $a=v^2/r$. 

In response to your query about taking components parallel and perpendicular to the incline, these are $W\sin\theta$ and $N-W\cos\theta$ (see diagram below).

The resultant R must be horizontal, because the condition is that the car moves in a horizontal circle. So
$\frac{N-W\cos\theta}{W\sin\theta}=\tan\theta=\frac{\sin\theta}{\cos\theta}$
$N\cos\theta-W\cos^2\theta=W\sin^2\theta$
$N\cos\theta=W(\sin^2\theta+\cos^2\theta)=W$.  
The magnitude of the resultant force R is such that
$R^2=(N-W\cos\theta)^2+(W\sin\theta)^2=N^2-2WN\cos\theta+W^2=N^2-W^2$
$=N^2-N^2\cos^2\theta=N^2\sin^2\theta$
$R=N\sin\theta$
as before.
This calculation was much more difficult than in the 1st diagram. Which shows how much simpler problems can be when you choose an appropriate co-ordinate system.
