To add station equation select Geometry>Horizontal Curve Set>Stationing
Select Horizontal Alignment to be edited
If you have Vertical Alignment attached to your Horizontal, selecting Synchronize Starting Station will change the vertical alignment corresponding to the new stationing.
When entering the ahead station, you must specify a name for the station equation first. An alpha character must be included in the ahead station to differentiate the ahead station from the back station.
For example, suppose you want to define station 12+00.000 back to be 13+00.000 ahead. You would enter the back station as 12+00 and the ahead station as a13+00. Or if you were starting a new phase of your project (maybe) you could use your target button to identify the station or northing/easting, and key-in the new ahead station (a0+00). Select Apply, your Station Equation is now populated, and Apply.
If there are another station equation define the ahead station by adding alpha character such as "b" prefix to the station, "b15+000"