how do i set position on a slider?

[Dan Royer (Marginally Clever Robots)]

I'm building a simulation of a leg on a vertical slider like the image below.

A vertical slider to a hip box with a hinge to a thigh box to a hinge to a calf box.  The slider's position is the floor.  I expected to start with a slider.getPosition of ~28. instead it is set to zero. Is there some way to set the position to my expected value, or change this behavior? I also tried to move the hip to the slider center, set the axis, then move the hip back to get some distance. No dice. So.... what's the "right" way to do this, please?