In the past, one way to convert an Alignment to a linestring was to use the Offset Line command and enter a 0 offset and an Elevation Delta of 0 and that would create a linestring representation of the alignment, however because an alignment when offset can have a different length because of the curves at the offset length being longer or shorter than the source alignment, and because elements like Spirals in the source alignment when offset cannot be represented by a spiral, the only way to represent the 3D alignment as a linestring is to chord it as needed in the unsupported element types to create an approximation of the source line.
I was talking to Thomas from Byard Construction today and he raised the issue when I suggested that good practice for Site type Road Modeling using Vertical Design that I recommend converting alignments to linestrings first because then you can edit them and modify them as needed for the Vertical Design process.
We added the ability to convert an Alignment to a linestring using the Convert to Linestring command. This works perfectly for site roads where you are not offsetting the alignment, and where the alignment does not contain Spirals or Asymmetrical Vertical Curves.
We then got talking about what happens when you break a linestring that has a vertical curve around the break point. TBC will handle this by actually holding the PC or PT and PI points that now lie outside the length of the linestring that you can see graphically. If you turn on the line markings, you can see a VPI indicator that is not on the line. So it actually retains the geometry required to be able to recreate the true vertical geometry.
So by converting Alignments to Linestrings, and this really applies to Urban Street or Site / Subdivision Projects is a really good way to manage the centerlines, especially if you need to manipulate them for the purposes of creating a great surface model.
Hope this is helpful