Force Positive is an option that can be selected when specifying the Rotate About parameter.

It only has an effect when used with a negative cant. With a positive cant, the result is the same as using 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. This is something that can happen even with positive cant if you are using any other method than Inside Rail.