When you create cant, you've got several methods for the Rotate About parameter.

One is Force Positive. I assume the others are clearly explicit.
But, as I did, you may have questions about Force Positive.

The effect is visible when using negative cant.
With positive cant, the result is the same using Force Positive and Inside Rail 

The intent is to always move the rail up and never down. The datum is the track profile.

Not using Force Positive with negative cant will result in a rail lower than the profile. Something that can happen even with positive cant if you are using any other method than Inside Rail.