how would you layout a spiral(transitional) curve?

Answer 1

Well you have an alignment in, and one out.

That is what governs the layout, right?

Assume the spiral begins when going ahead on the alignment.

A spiral has an increasing curvature. It will go from no curvature to a maximum. Thus the curvature is a function of position. this is key, and how you model it.

You have to know how a spiral will deviate from a HZ curve. There is no PC, PT or PI like for a HZ curve.

You have to know the change in curvature per change in station.