[300SLR]

"Positive polarity" most often refers to which way the current passes through the controller. This is important with electronic controllers but doesn't matter with resistance controllers.
Which direction the car travels is completely separate.

From what you say, I think your track is wired to positive polarity.
If I understand the description correctly (and I'm not sure about that) it sounds like the track is wired for the normal direction of travel.
I might have misunderstood the description - but are sure the red and black wires on the motor are the right way round?

 

[300SLR]

Agreed - having the motor rotating the right way to take advantage of commutator advance is important.
(Of course that only matters if the motor has commutator advance - A lot of low end motors symmetrically timed, so they work equally well either way round)

The important thing to understand is that the direction of motor rotation is all about the mechanical arrangement. Let's look at the detail

The direction of motor rotation can be reversed by fitting the pinion on the other end of the motor (swap can end drive for endbell drive to can end drive or vica versa) Obviously this is only practical if there is enough shaft length at each end of the motor, and the motor will fit the chassis the other way round.

In an inline, putting the crown wheel on the other side of the pinion reverses direction of motor rotation.

In an anglewinder, which way round the chassis is built defines the gear position so swaping can end drive / endbell drive is the the only way to reverse the motor rotation.

A sidewinder is the same as an anglewinder, unless it possible to mount the gear on opposite side of the chassis - which reverses direction of motor rotation.

Obviously the lead wires need to be wired the correct way round so the car goes forward.