There is an enhancement request already created to be able to create the existing centerline from rails in addition (or instead of) to the regression line and points.
Note that the regression line with a scale 0 is the existing centerline. So it is not required to create the existing centerline as such.
Meanwhile you will find here a possible workflow. Just few minutes to get this existing centerline.
Considering it is a single initial task per track for a project, I think it is an acceptable workflow.
Then you can project it to the proposed vertical and annotate the level difference between the proposed vertical and existing centerline (or regression line) to get the lifts annotation.
If you set the location to the points of the existing centerline, you will get annotation at regression points location.
If you want even interval, then set the location based on the horizontal alignment
Here is an alternate workflow using Single Offset Entire Element instead of transform for the horizontal.
Not better or quicker, just different. The video also shows a report of horizontal and vertical offsets between new and existing.
Note this related article: How to report Smoothing Radius - OpenRail Wiki - OpenRail - Bentley Communities
It also demonstrates that even the regression line shows the slew (with exaggeration), the line itself runs through the survey points/regression points. The offset is just a "display mode".
It means you can also do a Copy Parallel (MicroStation tool) to create 2 lines that represent the maximum desirable slew, based on the scale used for the regression line.
Example:
You can see this in the video: magenta dashed lines both side of the track.